Cellulose fiber-containing composition and sheet

ABSTRACT

It is an object of the present invention to provide a composition comprising cellulose fibers having a fiber width of 1000 nm or less, the properties of which are maintained even after long-term storage. The present invention relates to a cellulose fiber-containing composition comprising cellulose fibers having a fiber width of 1000 nm or less and an antiseptic containing at least one type selected from a nitrogen atom, a sulfur atom and a halogen atom, wherein the content of the antiseptic is 10 parts by mass or less with respect to 100 parts by mass of the cellulose fibers

TECHNICAL FIELD

The present invention relates to a cellulose fiber-containingcomposition and sheet. More specifically, the present invention relatesto a composition and a sheet, both comprising ultrafine cellulosefibers.

BACKGROUND ART

In recent years, because of enhanced awareness of alternatives topetroleum resources and environmental consciousness, there has been afocus on materials utilizing reproducible natural fibers. Among naturalfibers, cellulose fibers having a fiber diameter of 10 μm or more and 50μm or less, in particular, wood-derived cellulose fibers (pulp) havebeen widely used mainly as paper products so far.

Ultrafine cellulose fibers, which have a fiber diameter of 1 μm or less,have also been known as cellulose fibers. In recent years, a sheetcomposed of such ultrafine cellulose fibers, and a complex comprising anultrafine cellulose fiber-containing sheet and a resin, have beendeveloped. Since ultrafine cellulose fibers can exhibit thickeningaction, the use of such ultrafine cellulose fibers as a thickener forvarious intended uses has also been studied.

For example, Patent Document 1 discloses a thickener comprisingultrafine cellulose fibers having a fiber width of 1000 nm or less, inwhich some hydroxy groups of cellulose constituting the fibers aresubstituted with phosphorus oxoacid groups. Patent Document 1 describesthat a composition comprising the thickener may comprise an antisepticas a functional additive, and exemplifies paraben such an antiseptic.Patent Document 2 discloses an aqueous ink composition comprisingcellulose fibers having a number average fiber width of 2 to 150 nm, atleast one of a coloring agent and a masking agent, and water. Inaddition, Patent Document 3 discloses an aqueous agrochemicalcomposition comprising cellulose fibers having a number average fiberwidth of 2 to 150 nm, an agrochemical active ingredient, and water. Inthese documents, an antiseptic is used as an additive, and1,2-benzisothiazolin-3-one is used.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-2015-189698-   Patent Document 2: JP-A-2013-181167-   Patent Document 3: JP-A-2011-57571

SUMMARY OF INVENTION Object to be Solved by the Invention

By the way, in some cases, a composition comprising cellulose fibershaving a fiber width of 1000 nm or less is stored as a liquidcomposition for a long period of time, depending on the mode of use. Thecomposition is required to maintain the viscosity of a liquidcomposition even after long-term storage.

Hence, in order to achieve the object of the conventional art, thepresent inventors have conducted studies for the purpose of providing acellulose fiber-containing composition that maintains the viscositythereof before storage, even after long-term storage.

Means for Solving the Object

As a result of intensive studies directed towards achieving theaforementioned object, the present inventors have found that anantiseptic having a predetermined structure is added in a predeterminedamount to a composition comprising cellulose fibers having a fiber widthof 1000 nm or less, so that the viscosity of the cellulosefiber-containing composition can be maintained even after long-termstorage.

Specifically, the present invention has the following configurations.

[1] A cellulose fiber-containing composition comprising cellulose fibershaving a fiber width of 1000 nm or less, and an antiseptic containing atleast one type selected from a nitrogen atom, a sulfur atom and ahalogen atom, wherein

the content of the antiseptic is 10 parts by mass or less with respectto 100 parts by mass of the cellulose fibers.

[2] The cellulose fiber-containing composition according to [1], whereinthe cellulose fibers have a phosphoric acid group or a phosphoric acidgroup-derived substituent.[3] The cellulose fiber-containing composition according to [1] or [2],wherein the antiseptic is at least one type selected from an organicnitrogen sulfur compound, an organic nitrogen halogen compound, anorganic nitrogen compound, and an organic sulfur compound.[4] The cellulose fiber-containing composition according to any one of[1] to [3], which further comprises a hydrophilic polymer, wherein

the content of the hydrophilic polymer is 0.5 parts by mass or more and50 parts by mass or less, with respect to 100 parts by mass of thecellulose fibers.

[5] The cellulose fiber-containing composition according to any one of[1] to [4], wherein the cellulose fiber-containing composition is aslurry, and when the solid concentration of the slurry is set at 1.8% bymass, the viscosity measured under conditions of 25° C. and a rotationnumber of 3 rpm is 70000 mPa·s or more.[6] The cellulose fiber-containing composition according to any one of[1] to [5], wherein the cellulose fiber-containing composition is aslurry having a viscosity retention percentage of 80% or more, whereinthe viscosity retention percentage is defined according to the followingequation:

Viscosity retention percentage (%)=slurry viscosity left at rest at 50°C. for 480 hours/initial slurry viscosity×100,

provided that the slurry viscosity after being left at rest at 50° C.for 480 hours is a viscosity obtained by setting the solid concentrationof the slurry at 1.8% by mass and measuring under conditions of 25° C.and a rotation number of 3 rpm, after the slurry has been left at restat 50° C. for 480 hours, whereas the initial slurry viscosity is aviscosity obtained by setting the solid concentration of the slurry at1.8% by mass and measuring under conditions of 25° C. and a rotationnumber of 3 rpm.

[7] The cellulose fiber-containing composition according to any one of[1] to [6], wherein the cellulose fiber-containing composition is aslurry, and the haze measured by setting the solid concentration of theslurry at 0.2% by mass and using a glass cell having an optical pathlength of 1 cm in accordance with JIS K 7136 is 2.0% or less.[8] The cellulose fiber-containing composition according to any one of[1] to [7], wherein the cellulose fiber-containing composition is aslurry having a haze increasing percentage of 50% or less, wherein thehaze increasing percentage is defined according to the followingequation:

Haze increasing percentage (%)=(haze of slurry after being left at restat 50° C. for 480 hours−initial haze of slurry)/initial haze ofslurry×100,

provided that the haze of a slurry after being left at rest at 50° C.for 480 hours is a haze measured by setting the solid concentration ofthe slurry at 0.2% by mass and using a glass cell having an optical pathlength of 1 cm in accordance with JIS K 7136, after the slurry has beenleft at rest at 50° C. for 480 hours, whereas the initial haze of aslurry is a haze measured by setting the solid concentration of theslurry at 0.2% by mass and using a glass cell having an optical pathlength of 1 cm in accordance with JIS K 7136.

[9] The cellulose fiber-containing composition according to any one of[1] to [8], wherein the cellulose fiber-containing composition is aslurry for use in the formation of a sheet.[10] The cellulose fiber-containing composition according to any one of[1] to [9], wherein the cellulose fiber-containing composition is aslurry for use in the formation of a sheet, wherein the sheet, thatformed after the slurry has been left at rest at 50° C. for 192 hours,has a haze measured in accordance with JIS K 7136 of 1% or less.[11] A sheet comprising cellulose fibers having a fiber width of 1000 nmor less and an antiseptic containing at least one type selected from anitrogen atom, a sulfur atom and a halogen atom, wherein

the content of the antiseptic is 10% by mass or less with respect to thetotal mass of the sheet.

[12] The sheet according to [11], wherein the tensile elastic modulusmeasured under conditions of 23° C. and a relative humidity of 50% inaccordance with JIS P 8113 is 3 GPa or more.

Advantageous Effects of Invention

According to the present invention, a cellulose fiber-containingcomposition that maintains the viscosity thereof before storage, evenafter long-term storage, can be obtained. The cellulose fiber-containingcomposition of the present invention is a composition that can be storedfor a long period of time.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between the amount of NaOHadded dropwise to a fiber raw material and the electrical conductivity.

FIG. 2 is a graph showing the relationship between the amount of NaOHadded dropwise to a fiber raw material having a carboxyl group and theelectrical conductivity.

EMBODIMENTS OF CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail. Thebelow-mentioned constituent features will be explained based onrepresentative embodiments or specific examples in some cases. However,the present invention is not limited to such embodiments.

(Cellulose Fiber-Containing Composition)

The present invention relates to a cellulose fiber-containingcomposition comprising cellulose fibers having a fiber width of 1000 nmor less (hereinafter also referred to as “ultrafine cellulose fibers”)and an antiseptic containing at least one type selected from a nitrogenatom, a sulfur atom and a halogen atom. Herein, the content of theantiseptic is 10 parts by mass or less with respect to 100 parts by massof the cellulose fibers. Since the composition of the present inventionis a composition comprising ultrafine cellulose fibers, it may also bereferred to as an “ultrafine cellulose fiber-containing composition.”

Since the cellulose fiber-containing composition of the presentinvention has the above-described configuration, it is a compositionthat can be stored for a long period of time. Specifically, even in acase where the cellulose fiber-containing composition of the presentinvention is stored over a long period of time, the properties of acomposition comprising ultrafine cellulose fibers are maintained.Specifically, even after long-term storage, the cellulosefiber-containing composition of the present invention can exhibitviscosity equivalent to that before the storage. In addition, thecellulose fiber-containing composition of the present invention can alsomaintain transparency equivalent to that before the storage.

Since the cellulose fiber-containing composition of the presentinvention comprises a predetermined amount of an antiseptic containingat least one type selected from a nitrogen atom, a sulfur atom and ahalogen atom, its initial viscosity before storage is also high. Hence,the composition itself can be used as a thickener. Moreover, thecellulose fiber-containing composition comprising an antisepticcontaining at least one type selected from a nitrogen atom, a sulfuratom and a halogen atom is also advantageous in terms of high safety andgood handling ability.

The content of the antiseptic is 10 parts by mass or less with respectto 100 parts by mass of the cellulose fibers, and it is preferably 5parts by mass or less, more preferably 3 parts by mass or less, evenmore preferably 1 part by mass or less, further preferably 0.5 parts bymass or less, particularly preferably 0.2 parts by mass or less, andmost preferably 0.1 part by mass or less. On the other hand, even in acase where the antiseptic is added to the cellulose fiber-containingcomposition in a trace amount, such as 0.05 parts by mass or less, or0.01 part by mass or less, with respect to 100 parts by mass of thecellulose fibers, it can exhibit its effects. In the present invention,even in a case where the content of the antiseptic is suppressed withinthe above-described range, stability can be enhanced upon long-termstorage, and the occurrence of inconveniences, such as a decrease inviscosity or a decrease in transparency due to changes with the passageof time, can be prevented. In addition, in the present invention, bysetting the content of the antiseptic within the above-described range,the initial viscosity before the storage of the composition can beenhanced. Besides, it may be adequate, if the antiseptic is added, evenin a trace amount, into the cellulose fiber-containing composition. Theminimum value of the content of the antiseptic may be set, for example,at 0.0001 part by mass. The content of the antiseptic in the cellulosefiber-containing composition can be quantified by spectroscopicmeasurements such as NMR or IR, MS fragment analysis, or UV analysis.

The form of the cellulose fiber-containing composition is notparticularly limited, and can be present in various forms such aspowders, jelly, slurry, or a solid. Among others, the cellulosefiber-containing composition is preferably a slurry, and more preferablya high-viscosity slurry. Specifically, the cellulose fiber-containingcomposition is a slurry having a solid concentration of 1.8% by mass,and also, preferably having a viscosity of 70000 mPa·s or more, whereinthe viscosity is measured under conditions of 25° C. and a rotationnumber of 3 rpm, more preferably having a viscosity of 73000 mPa·s ormore, further preferably having a viscosity of 75000 mPa·s or more, andparticularly preferably having a viscosity of 80000 mPa·s or more. Theabove-described viscosity is a viscosity before the storage of thecellulose fiber-containing composition, and in the present description,such a viscosity is also referred to as an “initial viscosity” at times.It is to be noted that the initial viscosity is a viscosity obtained byproducing a slurry, leaving it at room temperature, and then measuringthe viscosity thereof within 20 hours.

For the measurement of the initial viscosity, a type B viscometer (No. 4roter) (manufactured by BROOKFIELD; analog viscometer T-LVT) can beused. The measurement conditions are determined to be 25° C., a rotationnumber of 3 rpm, and a measurement time of 3 minutes.

When the cellulose fiber-containing composition of the present inventionis a slurry, the viscosity retention percentage defined according to thefollowing equation is preferably 80% or more, more preferably 90% ormore, and further preferably 95% or more. In addition, the viscosityretention percentage may be 100%, or may also exceed 100%.

Viscosity retention percentage (%)=slurry viscosity left at rest at 50°C. for 480 hours/initial slurry viscosity×100.

Herein, the slurry viscosity after being left at rest at 50° C. for 480hours is a viscosity obtained by setting the solid concentration of theslurry at 1.8% by mass, leaving the slurry at rest at 50° C. for 480hours, and then measuring the viscosity using a type B viscometer (No. 4roter) (manufactured by BROOKFIELD; analog viscometer T-LVT) underconditions of 25° C., a rotation number of 3 rpm, and a measurement timeof 3 minutes. On the other hand, the initial slurry viscosity is aviscosity obtained by setting the solid concentration of a slurry at1.8% by mass, producing the slurry, then leaving the produced slurry atroom temperature, and then measuring the viscosity within 20 hours usinga type B viscometer (No. 4 roter) (manufactured by BROOKFIELD; analogviscometer T-LVT) under conditions of 25° C., a rotation number of 3rpm, and a measurement time of 3 minutes.

When the cellulose fiber-containing composition of the present inventionis a slurry, the haze of the slurry, which is measured by setting thesolid concentration of the slurry at 0.2% by mass and using a glass cellhaving an optical path length of 1 cm in accordance with JIS K 7136, ispreferably 2.0% or less, more preferably 1.5% or less, and furtherpreferably 1.0% or less. Since the slurry of the cellulosefiber-containing composition of the present invention, which is in theabove-described range, is preferable because it can be used to form asheet having high transparency. The above-described haze value is a hazevalue before the storage of the cellulose fiber-containing composition,and in the present description, it is referred to as an “initial haze”in some cases. The initial haze is a haze measured within 20 hours aftera slurry has been produced and has been then left at room temperature.

Herein, the haze of a slurry is measured by setting the solidconcentration of the slurry at 0.2% by mass and using a glass cellhaving an optical path length of 1 cm in accordance with JIS K 7136.Specifically, the haze can be measured using a hazemeter (manufacturedby MURAKAMI COLOR RESEARCH LABORATORY Co., Ltd.; HM-150). As a glasscell having an optical path length of 1 cm, for example, a glass cellfor liquid having an optical path length of 1 cm (manufactured byFujiwara Scientific Co., Ltd.; MG-40; inverse optical path) can be used.The zero point is measured with ion exchange water which is placed inthe glass cell.

When the cellulose fiber-containing composition of the present inventionis a slurry, the haze increasing percentage defined according to thebelow-mentioned equation is preferably 50% or less, more preferably 30%or less, and further preferably 20% or less. The haze increasingpercentage may be 0%, or may also be a numerical value lower than 0%.

Haze increasing percentage (%)=(haze of slurry after being left at restat 50° C. for 480 hours−initial haze of slurry)/initial haze ofslurry×100

Herein, the haze of a slurry after being left at rest at 50° C. for 480hours is a haze value obtained by setting the solid concentration of theslurry at 0.2% by mass, and then measuring it using a glass cell havingan optical path length of 1 cm in accordance with JIS K 7136, after theslurry has been left at rest at 50° C. for 480 hours. On the other hand,the initial haze of a slurry is a haze value obtained by setting thesolid concentration of the slurry at 0.2% by mass, and then measuring itusing a glass cell having an optical path length of 1 cm in accordancewith JIS K 7136. As an apparatus for measuring the haze, a hazemeter(manufactured by MURAKAMI COLOR RESEARCH LABORATORY Co., Ltd.; IM-150)can be used.

Moreover, when the cellulose fiber-containing composition is a slurry,the total transmittance obtained by setting the solid concentration ofthe slurry at 0.2% by mass, and then measuring it using a glass cellhaving an optical path length of 1 cm in accordance with JIS K 7136, ispreferably 90% or more, more preferably 93% or more, and furtherpreferably 95% or more. The above-described total light transmittance isa total light transmittance before the storage of the cellulosefiber-containing composition, and in the present description, the totallight transmittance before the storage of the cellulose fiber-containingcomposition is referred to as an “initial total light transmittance” insome cases. The initial total light transmittance is a total lighttransmittance measured within 20 hours after the slurry has beenproduced and has been then left at room temperature.

Herein, the total light transmittance of a slurry is measured by settingthe solid concentration of the slurry at 0.2% by mass and using a glasscell having an optical path length of 1 cm in accordance with JIS K7136. Specifically, the measurement can be carried out using a hazemeter(manufactured by MURAKAMI COLOR RESEARCH LABORATORY Co., Ltd.; HM-150).As a glass cell having an optical path length of 1 cm, for example, aglass cell for liquid having an optical path length of 1 cm(manufactured by Fujiwara Scientific Co., Ltd.; MG-40; inverse opticalpath) can be used. The zero point is measured with ion exchange waterwhich is placed in the glass cell.

When the cellulose fiber-containing composition of the present inventionis a slurry, the retention percentage of the total light transmittanceof the slurry, which is defined according to the below-mentionedequation, is preferably 80% or more, more preferably 90% or more, andfurther preferably 95% or more. The retention percentage of the totallight transmittance may be 100%, or may also exceed 100%.

Retention percentage (%) of total light transmittance=total lighttransmittance of slurry after being left at rest at 50° C. for 480hours/initial total light transmittance of slurry×100

Herein, the total light transmittance of the slurry after being left atrest at 50° C. for 480 hours is a total light transmittance obtained bysetting the solid concentration of the slurry at 0.2% by mass, and thenmeasuring it using a glass cell having an optical path length of 1 cm inaccordance with JIS K 7136, after the slurry has been left at rest at50° C. for 480 hours. On the other hand, the initial total lighttransmittance of the slurry is a total light transmittance obtained bysetting the solid concentration of the slurry at 0.2% by mass, and thenmeasuring it using a glass cell having an optical path length of 1 cm inaccordance with JIS K 7136. As an apparatus for measuring the totallight transmittance, a hazemeter (manufactured by MURAKAMI COLORRESEARCH LABORATORY Co., Ltd.; HM-150) can be used.

The intended use of the cellulose fiber-containing composition of thepresent invention is not limited, but it can be used as a thickener forvarious intended uses. Moreover, the present cellulose fiber-containingcomposition can also be used as a reinforcing material by being mixedwith a resin or an emulsion. Moreover, a slurry of the cellulosefiber-containing composition may be molded to produce a molded body. Inthis case, it is preferable to mix a resin or an emulsion into theslurry of the cellulose fiber-containing composition. The molded bodymay have various shapes other than the shape of a sheet.

In the present invention, the slurry of the cellulose fiber-containingcomposition may be used to form a film, so that various types of sheetsmay be produced. Among others, the cellulose fiber-containingcomposition of the present invention is preferably a slurry for use insheet formation. Since the cellulose fiber-containing composition of thepresent invention is a slurry having high transparency, it is preferablyused in formation of a sheet.

When the cellulose fiber-containing composition of the present inventionis a slurry for use in sheet formation, it is preferably a slurry foruse in sheet formation, which can be used to form a highly-transparentsheet, even after the slurry has been stored for a long period of time.Specifically, even in a case where the slurry for use in sheet formationis left at rest at 50° C. for 192 hours, it can be used to form a sheethaving a haze measured in accordance with JIS K 7136, which ispreferably 10% or less, more preferably 5% or less, and furtherpreferably 1% or less.

A cellulose fiber-containing composition having a solid form, such asjelly or powders, can be obtained, for example, by concentrating theslurry of the cellulose fiber-containing composition. In addition, thecellulose fiber-containing composition, which is in the form of powders,is obtained, for example, by pulverizing the condensate obtained byconcentration as described above. Thus, by processing the cellulosefiber-containing composition into a solid form such as jelly or powders,the water content in the powders can be reduced. Hence, the cellulosefiber-containing composition having a solid form is advantageous in thatit is easily mixed with a hydrophobic resin. Moreover, by dispersing thecellulose fiber-containing composition having a solid form such as jellyor powders in a solvent such as water, so as to prepare a slurry again,and transport costs can be advantageously reduced.

The method of concentrating a slurry is not particularly limited, and acondensate of the slurry can be obtained by any one of filtration,compression and drying, or by a combination thereof.

Filtration can be carried out by a common method such as naturalfiltration, vacuum filtration, pressure filtration or a centrifugalfiltration, but the method is not particularly limited. The usedfiltration material is not particularly limited, and a stainless steelmaterial, a filter paper, a polypropylene material, a nylon material, apolyethylene material, a polyester material, or the like can be used.

Compression can be carried out using a common press apparatus such as abelt press, a screw press, or a filter press, but the method is notparticularly limited. The used filtration material is not particularlylimited, and a stainless steel material, a filter paper, a polypropylenematerial, a nylon material, a polyethylene material, a polyestermaterial, or the like can be used.

Drying can be carried out by a common method such as hot air drying,cold air drying, freeze-drying, spray drying, microwave drying, or highfrequency drying, but the method is not particularly limited.

Pulverization can be carried out using a common pulverization apparatus,such as a ball mill, a bead mill, a colloid mill, a conical mill, a diskmill, an edge mill, a milling mill, a hammer mill, a mortar, a pelletmill, a VS1 mill, Wiley mill, a windmill (milling machine), a rollermill, or a jet mill, but the method is not particularly limited.

As described above, the cellulose fiber-containing composition, which isin the form of a solid, is re-suspended in a solvent, so as to obtain are-dispersion of ultrafine cellulose fibers. The solvent used in there-suspension of ultrafine cellulose fibers is not particularly limited.Examples of such a solvent include water, a single use of organicsolvent, and a mixture of water and an organic solvent. Examples of theorganic solvent include alcohols, polyhydric alcohols, ketones, ethers,dimethyl sulfoxide (DMSO), dimethylformamide (DMF) anddimethylacetoamide (DMAc). The alcohols include methanol, ethanol,n-propanol, isopropanol, n-butanol and t-butyl alcohol. The polyhydricalcohols include ethylene glycol and glycerin. The ketones includeacetone and methyl ethyl ketone. The ethers include diethyl ether,tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, ethylene glycol mono-n-butyl ether and ethylene glycolmono-t-butyl ether. The organic solvents may be used in a single type,or concurrently in two or more types. Among these, the solvent ispreferably a mixture of alcohols and water, a mixture of ethers ofwater, or a mixture of DMSO and water.

(Cellulose Fibers)

The cellulose fiber-containing composition of the present inventioncomprises cellulose fibers having a fiber width of 1000 nm or less(ultrafine cellulose fibers). The ultrafine cellulose fibers arepreferably fibers having an ionic functional group, and in this case,the ionic functional group is preferably an anionic functional group(hereinafter also referred to as an “anionic group”). The anionic groupis preferably at least one selected from, for example, a phosphoric acidgroup or a phosphoric acid group-derived substituent (which is simplyreferred to as a “phosphoric acid group” at times), a carboxyl group ora carboxyl group-derived substituent (which is simply referred to as a“carboxyl group” at times), and a sulfone group or a sulfonegroup-derived substituent (which is simply referred to as a “sulfonegroup” at times); is more preferably at least one selected from aphosphoric acid group and a carboxyl group; and is particularlypreferably a phosphoric acid group. In the present description,cellulose fibers having a phosphoric acid group are also referred to as“phosphorylated ultrafine cellulose fibers,” at times.

The content of the ultrafine cellulose fibers is preferably 40% by massor more, more preferably 50% by mass or more, and further preferably 55%by mass or more, with respect to the total solid content of thecellulose fiber-containing composition. On the other hand, the contentof the ultrafine cellulose fibers is preferably 95% by mass or less.

Although there is no particular restriction on a cellulose fiber rawmaterial for obtaining ultrafine cellulose fibers, pulp is preferablyused from the viewpoint of availability and inexpensiveness. Examples ofthe pulp include wood pulp, non-wood pulp, and deinked pulp. Examples ofthe wood pulp include chemical pulps such as leaf bleached kraft pulp(LBKP), needle bleached kraft pulp (NBKP), sulfite pulp (SP), dissolvingpulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygenbleached kraft pulp (OKP). Further, included are, but not particularlylimited to, semichemical pulps such as semi-chemical pulp (SCP) andchemi-ground wood pulp (CGP); and mechanical pulps such as ground pulp(GP) and thermomechanical pulp (TMP, BCTMP). Examples of the non-woodpulp include, but not particularly limited to, cotton pulps such ascotton linter and cotton lint; non-wood type pulps such as hemp, wheatstraw, and bagasse; and cellulose isolated from ascidian, seaweed, etc.,chitin, and chitosan. As a deinked pulp, there is deinked pulp usingwaste paper as a raw material, but it is not particularly limitedthereto. The pulp of the present embodiment may be used singly, or incombination of two or more types. Among the above-listed pulp types,wood pulp and deinked pulp including cellulose are preferable from theviewpoint of easy availability. Among wood pulps, chemical pulp ispreferable because it has a higher cellulose content to enhance theyield of ultrafine cellulose fibers and decomposition of cellulose inthe pulp is mild at the time of ultrafine fiber formation (defibration)to yield ultrafine cellulose fibers having a long fiber length with ahigh aspect ratio. Among them, kraft pulp and sulfite pulp are mostpreferably selected. A sheet containing the ultrafine cellulose fibershaving a long fiber length with a high aspect ratio tends to exhibit ahigh strength.

The average fiber width of ultrafine cellulose fibers is 1000 nm or lessas observed with an electron microscope. The average fiber width ispreferably 2 nm or more and 1000 nm or less, more preferably 2 nm ormore and 100 nm or less, even more preferably 2 nm or more and 50 nm orless, and further preferably 2 nm or more and 10 nm or less, but is notparticularly limited thereto. When the average fiber width of ultrafinecellulose fibers is less than 2 nm, since they are dissolved in water ascellulose molecules, there appears tendency that the physical properties(strength, rigidity, and dimensional stability) as an ultrafinecellulose fiber are not expressed sufficiently. The ultrafine cellulosefiber is, for example, monofilament cellulose having a fiber width of1000 nm or less.

The measurement of a fiber width of an ultrafine cellulose fiber byelectron microscopic observation is carried out as follows. An aqueoussuspension of ultrafine cellulose fibers having a concentration of 0.05%by mass or more and 0.1% by mass or less is prepared, and the suspensionis casted onto a hydrophilized carbon film-coated grid as a sample forTEM observation. If the sample contains wide fibers, SEM images of thesurface of the suspension casted onto glass may be observed. The sampleis observed using electron microscope images taken at a magnification of1000×, 5000×, 10000×, or 50000× according to the widths of theconstituent fibers. However, the sample, the observation conditions, andthe magnification are adjusted so as to satisfy the followingconditions:

(1) A single straight line X is drawn in any given portion in anobservation image, and 20 or more fibers intersect with the straightline X.(2) A straight line Y, which intersects perpendicularly with theaforementioned straight line in the same image as described above, isdrawn, and 20 or more fibers intersect with the straight line Y.

The widths of the fibers intersecting the straight line X and thestraight line Y in the observation image meeting the above-describedconditions are visually read. 3 or more sets of images of surfaceportions, which are at least not overlapped, are thus observed, and thewidths of the fibers intersecting the straight line X and the straightline Y are read in the each image. At least 120 fiber widths (20fibers×2×3-120) are thus read. The average fiber width (which is simplyreferred to as a “fiber width” at times) of ultrafine cellulose fibersis an average value of the fiber widths thus read.

The fiber length of the ultrafine cellulose fibers is not particularlylimited, and it is preferably 0.1 μm or more and 1000 μm or less, morepreferably 0.1 μm or more and 800 μm or less, and particularlypreferably 0.1 μm or more and 600 μm or less. By setting the fiberlength within the above-described range, destruction of the crystallineregion of the ultrafine cellulose fibers can be suppressed, and theslurry viscosity of the ultrafine cellulose fibers can also be setwithin an appropriate range. It is to be noted that the fiber length ofthe ultrafine cellulose fibers can be obtained by an image analysisusing TEM, SEM or AFM.

The ultrafine cellulose fibers preferably have a type I crystalstructure. In this regard, the fact that ultrafine cellulose fibers havea type I crystal structure may be identified by a diffraction profileobtained from a wide angle X-ray diffraction photograph using CuKα(λ=1.5418 Å) monochromatized with graphite. Specifically, it may beidentified based on the fact that there are typical peaks at twopositions near 2θ=14° or more and 17° or less, and near 2θ=22° or moreand 23° or less.

The percentage of the type I crystal structure occupied in the ultrafinecellulose fibers is preferably 30% or more, more preferably 50% or more,and further preferably 70% or more. In this case, more excellentperformance can be expected, in terms of heat resistance and theexpression of low linear thermal expansion. The crystallinity can beobtained by measuring an X-ray diffraction profile and obtaining itaccording to a common method (Seagal et al., Textile Research Journal,Vol. 29, p. 786, 1959).

The ultrafine cellulose fibers preferably have phosphoric acid groups orsubstituents derived from the phosphoric acid group. The phosphoric acidgroup is a divalent functional group corresponding to a phosphoric acidfrom which a hydroxyl group is removed. Specifically, it is a grouprepresented by —PO₃H₂. The substituents derived from the phosphoric acidgroup include substituents, such as condensation-polymerized phosphoricacid groups, salts of phosphoric acid groups, and phosphoric acid estergroups, and they may be either ionic substituents or nonionicsubstituents.

In the present invention, the phosphoric acid group or the substituentderived from the phosphoric acid group may be a substituent representedby the following Formula (1):

In the Formula (1), a, b, m and n each independently represent anintegral number (provided that a=b×m); α and α′ each independentlyrepresent R or OR. R is a hydrogen atom, a saturated straight chainhydrocarbon group, a saturated branched chain hydrocarbon group, asaturated cyclic hydrocarbon group, an unsaturated straight chainhydrocarbon group, an unsaturated branched chain hydrocarbon group, anaromatic group, or a derivative group thereof; and β is a monovalent orhigher valent cation consisting of organic matter or inorganic matter.

<Phosphoric Acid Group Introduction Step>

The phosphoric acid group introduction step may be performed by allowingat least one selected from a compound having phosphoric acid groups andsalts thereof (hereinafter, referred to as a “phosphorylating reagent”or “Compound A”) to react with the fiber raw material includingcellulose. Such a phosphorylating reagent may be mixed into the fiberraw material in a dry or wet state, in the form of a powder or anaqueous solution. In another example, a powder or an aqueous solution ofthe phosphorylating reagent may be added into a slurry of the fiber rawmaterial.

The phosphoric acid group introduction step may be performed by allowingat least one selected from a compound having phosphoric acid groups andsalts thereof (a phosphorylating reagent or Compound A) to react withthe fiber raw material including cellulose. It is to be noted that thisreaction may be performed in the presence of at least one selected fromurea and derivatives thereof (hereinafter, referred to as “Compound B”).

One example of the method of allowing Compound A to act on the fiber rawmaterial in the presence of Compound B includes a method of mixing thefiber raw material in a dry or wet state with a powder or an aqueoussolution of Compound A and Compound B. Another example thereof includesa method of adding a powder or an aqueous solution of Compound A andCompound B to a slurry of the fiber raw material. Among them, a methodof adding an aqueous solution of Compound A and Compound B to the fiberraw material in a dry state, or a method of adding a powder or anaqueous solution of Compound A and Compound B to the fiber raw materialin a wet state is preferable because of the high homogeneity of thereaction. Compound A and Compound B may be added at the same time or maybe added separately. Alternatively, Compound A and Compound B to besubjected to the reaction may be first added as an aqueous solution,which may be then compressed to squeeze out redundant chemicals. Theform of the fiber raw material is preferably a cotton-like or thin sheetform, but the form is not particularly limited thereto.

The Compound A used in the present embodiment is at least one selectedfrom a compound having a phosphoric acid group and a salt thereof.

Examples of the compound having a phosphoric acid group include, but arenot particularly limited to, phosphoric acid, lithium salts ofphosphoric acid, sodium salts of phosphoric acid, potassium salts ofphosphoric acid, and ammonium salts of phosphoric acid. Examples of thelithium salts of phosphoric acid include lithium dihydrogen phosphate,dilithium hydrogen phosphate, trilithium phosphate, lithiumpyrophosphate, and lithium polyphosphate. Examples of the sodium saltsof phosphoric acid include sodium dihydrogen phosphate, disodiumhydrogen phosphate, trisodium phosphate, sodium pyrophosphate, andsodium polyphosphate. Examples of the potassium salts of phosphoric acidinclude potassium dihydrogen phosphate, dipotassium hydrogen phosphate,tripotassium phosphate, potassium pyrophosphate, and potassiumpolyphosphate. Examples of the ammonium salts of phosphoric acid includeammonium dihydrogen phosphate, diammonium hydrogen phosphate,triammonium phosphate, ammonium pyrophosphate, and ammoniumpolyphosphate.

Among them, from the viewpoints of high efficiency in introduction ofthe phosphoric acid group, an improving tendency of the defibrationefficiency in a defibration step described below, low cost, andindustrial applicability, phosphoric acid, sodium phosphate, potassiumphosphate, and ammonium phosphate are preferable. Sodium dihydrogenphosphate, or disodium hydrogen phosphate is more preferable.

Further, since the uniformity of the reaction is improved and theefficiency in introduction of a phosphoric acid group is enhanced, theCompound A is preferably used as an aqueous solution. Although there isno particular restriction on the pH of an aqueous solution of theCompound A, the pH is preferably pH 7 or less because the efficiency inintroduction of a phosphoric acid group is high, and more preferably pH3 or more and pH 7 or less from the viewpoint of suppression ofhydrolysis of a pulp fiber. The pH of an aqueous solution of theCompound A may be adjusted, for example, by using, among compoundshaving a phosphoric acid group, a combination of an acidic one and analkaline one, and changing the amount ratio thereof. The pH of anaqueous solution of Compound A may also be adjusted by adding aninorganic alkali or an organic alkali to an acidic compound amongcompounds having a phosphoric acid group.

Although there is no particular restriction on the amount of theCompound A added to a fiber raw material, if the amount of the CompoundA added is converted to a phosphorus atomic weight, the amount ofphosphorus atoms added with respect to the fiber raw material (absolutedry mass) is preferably 0.5% by mass or more and 100% by mass or less,more preferably 1% by mass or more and 50% by mass or less, and mostpreferably 2% by mass or more and 30% by mass or less. When the amountof phosphorus atoms added to the fiber raw material is within theabove-described range, the yield of ultrafine cellulose fibers can befurther improved. On the other hand, by setting the amount of phosphorusatoms added to the fiber raw material at 100% by mass or less, the costof the used Compound A can be suppressed, while enhancingphosphorylation efficiency.

Examples of the Compound B used in the present embodiment include urea,biuret, 1-phenyl urea, 1-benzyl urea, 1-methyl urea, 1-ethyl urea andthe like.

The Compound B is preferably used as an aqueous solution, as with theCompound A. Further, an aqueous solution in which both the Compound Aand Compound B are dissolved is preferably used, because the uniformityof a reaction may be enhanced. The amount of the Compound B added to afiber raw material (absolute dry mass) is preferably 1% by mass or moreand 500% by mass or less, more preferably 10% by mass or more and 400%by mass or less, further preferably 100% by mass or more and 350% bymass or less, and particularly preferably 150% by mass or more and 300%by mass or less.

The reaction system may contain an amide or an amine, in addition to theCompound A and the Compound B. Examples of the amide include formamide,dimethylformamide, acetamide, and dimethylacetamide. Examples of theamine include methylamine, ethylamine, trimethylamine, triethylamine,monoethanolamine, diethanolamine, triethanolamine, pyridine,ethylenediamine, and hexamethylenediamine. Among them, particularly,triethylamine is known to work as a favorable reaction catalyst.

In the phosphoric acid group introduction step, it is preferable toperform a heat treatment. For the temperature of such a heat treatment,it is preferable to select a temperature that allows an efficientintroduction of phosphoric acid groups while suppressing the thermaldecomposition or hydrolysis reaction of fibers. Specifically, thetemperature is preferably 50° C. or higher and 300° C. or lower, morepreferably 100° C. or higher and 250° C. or lower, and furtherpreferably 130° C. or higher and 200° C. or lower. In addition, a vacuumdryer, an infrared heating device, or a microwave heating device may beused for heating.

Upon the heat treatment, if the time for leaving the fiber raw materialto stand still gets longer while the fiber raw material slurry to whichthe Compound A is added contains water, as drying advances, watermolecules and the Compound A dissolved therein move to the surface ofthe fiber raw material. As such, there is a possibility of theoccurrence of unevenness in the concentration of the Compound A in thefiber raw material, and the introduction of phosphoric acid groups tothe fiber surface may not progress unifomly. In order to suppress theoccurrence of unevenness in the concentration of the Compound A in thefiber raw material due to drying, the fiber raw material in the shape ofa very thin sheet may be used, or a method of heat-drying orvacuum-drying the fiber raw material, while kneading or stirring withthe Compound A using a kneader or the like, may be employed.

As a beating device used for heat treatment, a device capable of alwaysdischarging moisture retained by slurry or moisture generated by anaddition reaction of phosphoric acid groups with hydroxy groups of thefiber to the outside of the device system is preferable, and forexample, forced convection ovens or the like are preferable. By alwaysdischarging moisture in the device system, in addition to being able tosuppress a hydrolysis reaction of phosphoric acid ester bonds, which isa reverse reaction of the phosphoric acid esterification, acidhydrolysis of sugar chains in the fiber may be suppressed as well, andultrafine fibers with a high axial ratio can be obtained.

The time for heat treatment is, although affected by the heatingtemperature, preferably 1 second or more and 300 minutes or less, morepreferably 1 second or more and 1000 seconds or less, and furtherpreferably 10 seconds or more and 800 seconds or less, after moisture issubstantially removed from the fiber raw material slurry. In the presentinvention, by setting the heating temperature and heating time within anappropriate range, the amount of phosphoric acid groups introduced canbe set within a preferred range.

The amount of phosphoric acid groups introduced is, per 1 g (mass) ofthe ultrafine cellulose fibers, preferably 0.1 mmol/g or more and 3.65mmol/g or less, more preferably 0.14 mmol/g or more and 3.5 mmol/g orless, even more preferably 0.2 mmol/g or more and 3.2 mmol/g or less,particularly preferably 0.4 mmol/g or more and 3.0 mmol/g or less, andmost preferably 0.6 mmol/g or more and 2.5 mmol/g or less. By settingthe amount of phosphoric acid groups introduced within theabove-described range, it may become easy to perform fibrillation on thefiber raw material, and the stability of the ultrafine cellulose fiberscan be enhanced. In addition, by setting the amount of phosphoric acidgroups introduced within the above-described range, it becomes possibleto keep the hydrogen bond between ultrafine cellulose fibers, whilefacilitating fibrillation, and thus, the sheet is anticipated to havefavorable strength.

The amount of phosphoric acid groups introduced into a fiber rawmaterial may be measured by a conductometric titration method.Specifically, the amount introduced may be measured by performingfibrillation on ultrafine fibers in a defibration treatment step,treating the resulting slurry comprising ultrafine cellulose fibers withan ion exchange resin, and then examining a change in the electricalconductivity while adding an aqueous sodium hydroxide solution.

The conductometric titration confers a curve shown in FIG. 1 as analkali is added. First, the electrical conductivity is rapidly reduced(hereinafter, this region is referred to as a “first region”). Then, theconductivity starts rising slightly (hereinafter, this region isreferred to as a “second region”). Then, the increment of theconductivity is increased (hereinafter, this region is referred to as a“third region”). The boundary point between the second region and thethird region is defined as a point at which a change amount in the twodifferential values of conductivity, namely, an increase in theconductivity (inclination) becomes maximum. In short, three regionsappear. Among them, the amount of the alkali required for the firstregion among these regions is equal to the amount of a strongly acidicgroup in the slurry used in the titration, and the amount of the alkalirequired for the second region is equal to the amount of a weakly acidicgroup in the slurry used in the titration. When condensation of aphosphoric acid group occurs, the weakly acidic group is apparentlylost, so that the amount of the alkali required for the second region isdecreased as compared with the amount of the alkali required for thefirst region. On the other hand, the amount of the strongly acidic groupagrees with the amount of the phosphorus atom regardless of the presenceor absence of condensation. Therefore, the simple term “the amount ofthe phosphoric acid group introduced (or the amount of the phosphoricacid group)” or “the amount of the substituent introduced (or the amountof the substituent)” refers to the amount of the strongly acidic group.That is to say, the amount (mmol) of the alkali required for the firstregion in the curve shown in FIG. 1 is divided by the solid content (g)in the slurry as a titration target to obtain the amount (mmol/g) of thesubstituent introduced.

The phosphoric acid group introduction step may be performed at leastonce, but may be repeated multiple times as well. This case ispreferable, since more phosphoric acid groups are introduced.

<Introduction of Carboxyl Group>

In the present invention, when the ultrafine cellulose fibers havecarboxyl groups, such carboxyl groups can be introduced into theultrafine cellulose fibers, for example, by performing an oxidationtreatment such as a TEMPO oxidation treatment on the fiber raw material,or by treating the ultrafine cellulose fibers with a compound havinggroups derived from carboxylic acid, a derivative thereof, or an acidanhydride thereof or a derivative thereof.

Examples of the compound having a carboxyl group include, but are notparticularly limited to, dicarboxylic acid compounds such as maleicacid, succinic acid, phthalic acid, fumaric acid, glutaric acid, adipicacid or itaconic acid, and tricarboxylic acid compounds such as citricacid or aconitic acid.

Examples of the acid anhydride of the compound having a carboxyl groupinclude, but are not particularly limited to, acid anhydrides ofdicarboxylic acid compounds, such as maleic anhydride, succinicanhydride, phthalic anhydride, glutaric anhydride, adipic anhydride, oritaconic anhydride.

Examples of the derivative of the compound having a carboxyl groupinclude, but are not particularly limited to, an imidized product of theacid anhydride of the compound having a carboxyl group and a derivativeof the acid anhydride of the compound having a carboxyl group. Examplesof the imidized product of the acid anhydride of the compound having acarboxyl group include, but are not particularly limited to, imidizedproducts of dicarboxylic acid compounds, such as maleimide, succinimideor phthalimide.

The derivative of the acid anhydride of the compound having a carboxylgroup is not particularly limited. Examples include acid anhydrides ofthe compounds having a carboxyl group, in which at least some hydrogenatoms are substituted with substituents (for example, an alkyl group, aphenyl group, etc.), such as dimethylmaleic anhydride, diethylmaleicanhydride, or diphenylmaleic anhydride.

The amount of carboxyl groups introduced is, per 1 g (mass) of theultrafine cellulose fibers, preferably 0.1 mmol/g or more and 3.65mmol/g or less, more preferably 0.14 mmol/g or more and 3.5 mmol/g orless, further preferably 0.2 mmol/g or more and 3.2 mmol/g or less,particularly preferably 0.4 mmol/g or more and 3.0 mmol/g or less, andmost preferably 0.6 mmol/g or more and 2.5 mmol/g or less. The amount ofcarboxyl groups introduced into a fiber raw material can be measured bya conductometric titration method. In conductometric titration, additionof alkali gives the curve shown in FIG. 2. The amount of the alkali(mmol) required for the first region in the curve shown in FIG. 2 isdivided by the solid content (g) in the slurry to be titrated todetermine the amount of the substituents introduced (mmol/g).

<Alkali Treatment>

When ultrafine cellulose fibers are produced, an alkali treatment may beconducted between an ionic functional group introduction step and adefibration treatment step described below. The method of the alkalitreatment is not particularly limited. For example, a method ofimmersing functional group-introduced fibers in an alkaline solution maybe applied.

The alkali compound contained in the alkaline solution is notparticularly limited, but it may be an inorganic alkaline compound or anorganic alkali compound. The solvent of the alkaline solution may beeither water or an organic solvent. The solvent is preferably a polarsolvent (water, or a polar organic solvent such as alcohol), and morepreferably an aqueous solvent containing at least water.

Among alkaline solutions, a sodium hydroxide aqueous solution, or apotassium hydroxide aqueous solution is particularly preferable, becauseof high versatility.

The temperature of the alkali solution in the alkali treatment step isnot particularly limited, but it is preferably 5° C. or higher and 80°C. or lower, and more preferably 10° C. or higher and 60° C. or lower.

The immersion time in the alkali solution in the alkali treatment stepis not particularly limited, but it is preferably 5 minutes or more and30 minutes or less, and more preferably 10 minutes or more and 20minutes or less.

The amount of the alkali solution used in the alkali treatment is notparticularly limited, but it is preferably 100% by mass or more and100000% by mass or less, and more preferably 1000% by mass and 10000% bymass or less, with respect to the absolute dry mass of the phosphoricacid group-introduced fibers.

In order to reduce the consumption of an alkaline solution in the alkalitreatment step, functional group-introduced fibers may be washed withwater or an organic solvent before the alkali treatment step. After thealkali treatment, the alkali-treated functional group-introduced fibersare preferably washed with water or an organic solvent before thedefibration treatment step in order to improve the handling property.

<Defibration Treatment>

The ionic functional group-introduced fibers are subjected to adefibration treatment in a defibration treatment step. In thedefibration treatment step, fibers are defibrated usually using adefibration treatment apparatus to yield a slurry comprising ultrafinecellulose fibers, and there is no particular restriction on a treatmentapparatus, or a treatment method.

A high-speed defibrator, a grinder (stone mill-type crusher), ahigh-pressure homogenizer, an ultrahigh-pressure homogenizer, ahigh-pressure collision-type crusher, a ball mill, a bead mill, or thelike can be used as the defibration treatment apparatus. Alternatively,for example, a wet milling apparatus such as a disc-type refiner, aconical refiner, a twin-screw kneader, an oscillation mill, a homomixerunder high-speed rotation, an ultrasonic disperser, or a beater may alsobe used as the defibration treatment apparatus. The defibrationtreatment apparatus is not limited to the above. Examples of a preferreddefibration treatment method include a high-speed defibrator, ahigh-pressure homogenizer, and an ultrahigh-pressure homogenizer, whichare less affected by milling media, and are free from apprehension ofcontamination.

Upon the defibration treatment, the fiber raw material is preferablydiluted with water and an organic solvent each alone or in combination,to prepare a slurry, though the method is not particularly limitedthereto. Water as well as a polar organic solvent can be used as adispersion medium. Preferred examples of the polar organic solventinclude, but are not particularly limited to, alcohols, ketones, ethers,dimethyl sulfoxide (DMSO), dimethylformamide (DMF), anddimethylacetamide (DMAc). Examples of the alcohols include methanol,ethanol, n-propanol, isopropanol, n-butanol, and t-butyl alcohol.Examples of the ketones include acetone and methyl ethyl ketone (MEK).Examples of the ethers include diethyl ether and tetrahydrofuran (THF).One of these dispersion media may be used, or two or more thereof may beused. The dispersion medium may also contain a solid content other thanthe fiber raw material, for example, hydrogen-binding urea.

With regard to the ultrafine cellulose fibers, the ultrafine cellulosefiber-containing slurry obtained by the defibration treatment may beonce concentrated and/or dried, and then, may be subjected to adefibration treatment again. In this case, there is no particularrestriction on the method of concentration and drying, but examplesthereof include a method in which a concentrating agent is added into aslurry comprising ultrafine cellulose fibers, and a method using adehydrator, a press, a dryer, and the like used generally. Further,publicly known methods, for example as described in WO 2014/024876, WO2012/107642, and WO 2013/121086, may be used. Also, the ultrafinecellulose fiber-containing slurry may be formed into a sheet, so that itis concentrated and dried. The formed sheet is subjected to adefibration treatment, so that an ultrafine cellulose fiber-containingslurry can be obtained again.

Examples of a device used for defibrating (pulverizing) the ultrafinecellulose fiber-containing slurry again, after the concentration and/ordrying of the ultrafine cellulose fiber-containing slurry, include, butare not particularly limited to, a high-speed defibrator, a grinder(stone mill-type grinder), a high-pressure homogenizer, an ultra-highpressure homogenizer, a high-pressure collision type crusher, a ballmill, a bead mill, a disk type refiner, a conical refiner, a twin screwkneader, a vibrating mill, and a device for wet milling, such as ahigh-speed rotating homomixer, an ultrasonic disperser, or a beater.

(Antiseptic)

The cellulose fiber-containing composition of the present inventioncomprises an antiseptic containing at least one type selected from anitrogen atom, a sulfur atom and a halogen atom. Herein, the content ofthe antiseptic is 10 parts by mass or less with respect to 100 parts bymass of the cellulose fibers, and it is preferably 5 parts by mass orless, more preferably 3 parts by mass or less, even more preferably 1part by mass or less, further preferably 0.5 parts by mass or less,particularly preferably 0.2 parts by mass or less, and most preferably0.1 part by mass or less. On the other hand, the antiseptic may be addedto the cellulose fiber-containing composition in a trace amount, such as0.05 parts by mass or less, or 0.01 part by mass or less, with respectto 100 parts by mass of the cellulose fibers. Besides, the minimum valueof the content of the antiseptic can be set at, for example, 0.0001 partby mass.

The antiseptic comprises a solvent in some cases. In this case, acompound containing at least one type selected from a nitrogen atom, asulfur atom and a halogen atom is preferably comprised in an amount of0.1% by mass or more and 30% by mass or less, with respect to the totalmass of the antiseptic comprising a solvent.

The antiseptic containing at least one type selected from a nitrogenatom, a sulfur atom and a halogen atom comprises the compound containingat least one type selected from a nitrogen atom, a sulfur atom and ahalogen atom. Such a compound is preferably at least one type selectedfrom an organic nitrogen sulfur compound, an organic nitrogen halogencompound, an organic nitrogen compound, and an organic sulfur compound.The organic nitrogen sulfur compound is an organic compound containing anitrogen atom and a sulfur atom in a single molecule of the compound.The organic nitrogen halogen compound is an organic compound containingnitrogen and halogen atoms in a single molecule of the compound.Examples of such a halogen atom include a fluorine atom, a chlorineatom, a bromine atom, and an iodine atom. Among others, the halogen atomis preferably a bromine atom. The organic nitrogen compound is anorganic compound containing a nitrogen atom in a single molecule of thecompound. In the present description, the organic nitrogen compound is acompound that does not contain a sulfur atom and a halogen atom, and itis distinguished from the above-described organic nitrogen sulfurcompound or organic nitrogen halogen compound. In addition, the organicsulfur compound contains a sulfur atom in a single molecule of thecompound. It is to be noted that, in the present description, theorganic sulfur compound is a compound that does not contain a nitrogenatom, and that the organic sulfur compound is distinguished from theabove-described organic nitrogen sulfur compound.

The organic nitrogen sulfur compound, the organic nitrogen halogencompound, the organic nitrogen compound, and the organic sulfur compoundmay be either chain compounds, or cyclic compounds. When the compoundcomprises a cyclic structure, the cyclic structure is preferably aheteroring. For example, when the organic nitrogen sulfur compoundcomprises a cyclic structure, it preferably comprises, as a heteroatom,at least one type selected from a nitrogen atom and a sulfur atom. Whenthe organic nitrogen halogen compound and the organic nitrogen compoundcomprise a cyclic structure, these compounds preferably comprise aheteroring containing a nitrogen atom as a heteroatom. In addition, whenthe organic sulfur compound comprises a cyclic structure, it preferablycomprises a heteroring containing a sulfur atom as a heteroatom.

Examples of the organic nitrogen sulfur compound include: alkylenebisthiocyanates such as methylene bisthiocyanate or ethylenebisthiocyanate; 3-isothiazolone compounds and complexes of such3-isothiazolone compounds and metal salts, such as5-chloro-2-methyl-4-isothiazolin-3-one, 2-methyl-4-isothiazolin-3-one,4,5-dichloro-2-n-octyl-isothiazolin-3-one, 2-n-octyl-isothiazolin-3-one,or 1,2-benzisothiazolin-3-one; dithiocarbamate compounds, such asammonium N-methyldithiocarbamate, sodium N-methyldithiocarbamate,dimethyldithiocarbamate (sodium salt), ethylene thiuram monosulfide,disodium ethylene bisdithiocarbamate, or manganese ethylenebisdithiocarbamate; sulfonamide compounds, such as chloramine T orN,N-dimethyl-N′-(fluorodichloromethylthio)-N′-phenylsulfamide; thiazolecompounds, such as 2-(4-thiocyanomethylthio)benzothiazole or sodium2-mercaptobenzothiazole; and N-(fluorodichloromethylthio)-phthalimide,3,5-dimethyl-1,3,5-2H-tetrahydrothiadiazine-2-thione anddithio-2,2′-bis(benzmethylamde), and 2-pyridinethiol sodium oxide.

The organic nitrogen halogen compound is preferably an organic nitrogenbromine compound. Examples of the organic nitrogen bromine compoundinclude: organic bromonitro compounds, such as2-bromo-2-nitropropane-1,3-diol, 1,1-dibromo-1-nitro-2-propanol,2,2-dibromo-2-nitro-1-ethanol, 1,1-dibromo-1-nitro-2-acetoxyethane,1,1-dibromo-1-nitro-2-acetoxypropane,2-bromo-2-nitro-1,3-diacetoxy-propane, tribromonitromethane,β-bromo-β-nitrostyrene, 5-bromo-5-nitro-1,3-dioxane, or5-bromo-2-methyl-5-nitro-1,3-dioxane; organic bromcyano compounds, suchas 2,2-dibromo-3-nitrilopropionamide or2-bromo-2-bromomethyl-glutaronitrile; organic bromacetic acid esters oramides, such as 1,2-bis-(bromoacetoxy)-ethane,1,2-bis-(bromoacetoxy)-propane, 1,4-bis-(bromoacetoxy)-2-butene,1,2,3,-trisbromoacetoxypropane, methylenebis bromoacetate, benzylbromoacetate, or 2-bromoacetamide; organic bromosulfone compounds, suchas bistribromomethylsulfone; and 2-bromo-4′-hydroxyacetophenone,2,5-dichloro-4-bromophenol, 2,4,6-tribromophenol, α-bromocinnamaldehydeand 2-hydroxyethyl-2,3-dibromopropionate, and alkyl bromideisoquinolium.

Examples of the organic nitrogen halogen compound other than the organicnitrogen bromine compound include: alkyl (C8-C19) trimethyl ammoniumhalides, such as octyl trimethyl ammonium chloride, decyl trimethylammonium chloride, dodecyl trimethyl ammonium chloride, hexadecyltrimethyl ammonium chloride, or stearyltrimethyl ammonium chloride;dialkyl (C8-C18) dimethyl ammonium halides, such as octyldecyl dimethylammonium chloride, dioctyldimethyl ammonium chloride, didecyl dimethylammonium chloride, octyldodecyl dimethyl ammonium chloride, dihexadecyldimethyl ammonium chloride, or dioctadecyl dimethyl ammonium chloride;and alkyl (C8-C18) benzyldimethyl ammonium halides, such as dodecyldimethylbenzyl ammonium chloride, hexadecyl dimethylbenzyl ammoniumchloride, or octadecyl dimethylbenzyl ammonium chloride. Moreover,examples of the organic nitrogen halogen compound include: halogenatedoxime compounds, such as N,4-dihydroxy-α-oxobenzeneethane imidoylchloride, α-chloro-o-acetoxy benzaldoxime, or α,4-dichlorobenzaldoxime;amide compounds such as 2-chloroacetamide; chlorinated isocyanuric acidcompounds, such as dichloroisocyanate, sodium dichloroisocyanurate, ortrichloroisocyanuric acid; quaternary ammonium compounds, such asdequalinium chloride, or benzalkonium chloride; carbamic acids or estersthereof, such as 3-iodo-2-propargylbutyl carbamic acid; imidazolecompounds, such as1-[2-(2,4-dichlorophenyl)]-2′-[(2,4-dichlorophenyl)methoxy]ethyl-3-(2-phenylethyl)-1H-imidazoliumchloride, or1-[2-(2,4-dichlorophenyl)-2-(2-propenyloxy)ethyl]-1H-imidazolc; and2,4,5,6-tetrachloroisophthalonitrile,5-chloro-2,4,6,-trifluoroisophthalonitrile,5-chloro-2,4-difluoro-6-methoxyisophthalonitrile, andpoly[oxyethylene(dimethylimino)ethylene(dimethylimino)ethylenedichloride].

Examples of the organic nitrogen compound include: s-triazine compounds,such as hexahydro-1,3,5-tris-(2-ethyl)-s-triazine orhexahydro-1,3,5-tris-(2-hydroxyethyl)-s-triazine; carbamic acids oresters thereof, such as methyl 2-benzimidazolylcarbamate; amidecompounds such as 2-(2-furyl)-3-(5-nitro-2-furyl)-acrylic acid amide;aminoalcohol compounds, such as N-(2-hydroxypropyl)-aminomethanol or2-(hydroxymethylamino)ethanol; and N-(2-methyl-1-naphthyl)maleimide.

Examples of the organic sulfur compound include3,3,4,4-tetrachlorotetrahydrothiophene-1,1-dioxide and2-hydroxypropylmethane thiosulfonate.

Moreover, the cellulose fiber-containing composition of the presentinvention preferably comprises, as an antiseptic, only an antisepticcontaining at least one type selected from a nitrogen atom, a sulfuratom and a halogen atom. However, known antiseptics, such as3-acetoxy-1,1,2-triiodo-1-propene, glutardialdehyde, dichlorophene,hydrogen peroxide, and maleic anhydride, may also be used incombination, as necessary.

Among others, the antiseptic containing at least one type selected froma nitrogen atom, a sulfur atom and a halogen atom is preferably at leastone type selected from a compound having an isothiocyanate structure, acompound having a heteroring containing, as a heteroatom, at least onetype selected from a nitrogen atom and a sulfur atom, and abromonitroalcohol compound. By using such a compound, the long-termstorage of the cellulose fiber-containing composition becomes possible,and the viscosity and transparency equivalent to those before thestorage can be easily maintained, even though the additive amount of thecompound is small.

Specific examples of the above-described compounds will be given below.

The antiseptic containing at least one type selected from a nitrogenatom, a sulfur atom and a halogen atom may be produced by synthesis, buta commercially available product may also be used. For example, SLCseries (SLC795 and SLC755) and MLN series (MLN352, MLN460, and MLN553P),which are manufactured by KATAYAMA NALCO INC., can be used.

(Hydrophilic Polymer)

The cellulose fiber-containing composition of the present invention mayfurther comprise a hydrophilic polymer. In particular, when thecellulose fiber-containing composition is a slurry for use in sheetformation, it preferably comprises such a hydrophilic polymer. Byallowing the slurry for use in sheet formation to comprise a hydrophilicpolymer, an ultrafine cellulose fiber-containing sheet having hightransparency and high mechanical strength can be obtained.

Examples of the hydrophilic polymer include polyethylene glycol,cellulose derivatives (hydroxyethyl cellulose, carboxyethyl cellulose,carboxymethyl cellulose, etc.), casein, dextrin, starch, modifiedstarch, polyvinyl alcohol, modified polyvinyl alcohol (acetoacetylatedpolyvinyl alcohol, etc.), polyethylene oxide, polyvinylpyrrolidone,polyvinyl methyl ether, polyacrylates, polyacrylamide, acrylic acidalkyl ester copolymers, and urethane copolymers. Among others, thehydrophilic polymer is preferably at least one type selected frompolyethylene glycol (PEG), polyvinyl alcohol (PVA), modified polyvinylalcohol (modified PVA) and polyethylene oxide (PEO), and is morepreferably polyethylene oxide (PEO).

The content of the hydrophilic polymer is preferably 0.5 parts by massor more, more preferably 3 parts by mass or more, further preferably 5parts by mass or more, and particularly preferably 10 parts by mass ormore, with respect to 100 parts by mass of the cellulose fibers. On theother hand, the content of the hydrophilic polymer is preferably 50parts by mass or less, more preferably 40 parts by mass or less, andfurther preferably 30 parts by mass or less, with respect to 100 partsby mass of the cellulose fibers.

The viscosity-average molecular weight of the hydrophilic polymer is notparticularly limited, but it is preferably 1.0×10³ or more and 1.0×10⁷or less, more preferably 2.0×10³ or more and 1.0×10⁷ or less, andfurther preferably 5.0×10³ or more and 1.0×10⁷ or less.

Moreover, the cellulose fiber-containing composition of the presentinvention preferably further comprises, as a hydrophilic polymer,polyamine polyamide epihalohydrin. Polyamine polyamide epihalohydrin isa cationic thermosetting resin obtained by subjecting aliphatic dibasiccarboxylic acid or a derivative thereof and polyalkylene polyamine toheat condensation so as to synthesize polyamide polyamine, and thenallowing the polyamide polyamine to react with epihalohydrin. Since thepolyamine polyamide epihalohydrin is an aqueous resin, the polyaminepolyamide epihalohydrin may be added in the form of an aqueous resininto a slurry for use in sheet formation.

Examples of the polyamine polyamide epihalohydrin include polyaminepolyamide epichlorohydrin, polyamine polyamide epibromohydrin, andpolyamine polyamide epiiodohydrin. Among these, polyamine polyamideepichlorohydrin (PAE) is preferably used.

The content of the polyamine polyamide epihalohydrin is preferably 0.1part by mass or more, and more preferably 0.3 parts by mass or more,with respect to 100 parts by mass of the cellulose fibers. On the otherhand, the content of the polyamine polyamide epihalohydrin is preferably10 parts by mass or less, more preferably 5 parts by mass or less, andfurther preferably 3 parts by mass or less, with respect to 100 parts bymass of the cellulose fibers. In the case of adding such polyaminepolyamide epihalohydrin, the content of the polyamine polyamideepihalohydrin is not included in the content of the aforementionedhydrophilic polymer. By setting the content of the polyamine polyamideepihalohydrin within the above-described range, the water resistance andtransparency of the sheet can be enhanced.

(Thermoplastic Resin)

The cellulose fiber-containing composition of the present invention mayfurther comprise a thermoplastic resin. Examples of the thermoplasticresin include a styrene resin, an acrylic resin, an aromaticpolycarbonate resin, an aliphatic polycarbonate resin, an aromaticpolyester resin, an aliphatic polyester resin, an aliphatic polyolefinresin, a cyclic olefin resin, a polyamide resin, a polyphenylene etherresin, a thermoplastic polyimide resin, a polyacetal resin, apolysulfone resin, and an amorphous fluorine resin, but are not limitedthereto. Among others, a thermoplastic resin constituting thermoplasticresin fibers is preferably an aromatic polyester resin, an aliphaticpolyester resin, an aliphatic polyolefin resin, an aromaticpolycarbonate resin, or an aliphatic polycarbonate resin. Morespecifically, the thermoplastic resin is preferably at least one typeselected from, for example, polylactic acid, polybutylene succinate, anethylene-vinyl alcohol copolymer, amorphous PET, polypropylene,acid-modified polypropylene, polycarbonate and polyethylene, and is morepreferably at least one type selected from polypropylene andacid-modified polypropylene.

The thermoplastic resin may be a thermoplastic resin emulsion, but it ispreferably an emulsion. When the thermoplastic resin is a thermoplasticresin fiber, the fiber length (number average fiber length) of thethermoplastic resin fiber is preferably 0.5 mm or more, more preferably1 mm or more, and further preferably 3 mm or more. On the other hand,the fiber length (number average fiber length) of the thermoplasticresin fiber is preferably 100 mm or less, more preferably 50 mm or less,and further preferably 25 mm or less.

Moreover, the fiber diameter (number average fiber diameter) of thethermoplastic resin fiber is preferably 0.5 μm or more, more preferably1 μm or more, and further preferably 3 μm or more. On the other hand,the fiber diameter (number average fiber diameter) of the thermoplasticresin fiber is preferably 50 μm or less, more preferably 25 μm or less,and further preferably 15 μm or less.

The content of the thermoplastic resin is preferably 1 part by mass ormore, and more preferably 3 parts by mass or more, with respect to 100parts by mass of the cellulose fibers. On the other hand, the content ofthe thermoplastic resin is preferably 50 parts by mass or less, and morepreferably 30 parts by mass or less, with respect to 100 parts by massof the cellulose fibers. By adding the thermoplastic resin to thecellulose fiber-containing composition to result in the above-describedcontent, the sheet can have flexibility.

(Optional Component)

The cellulose fiber-containing composition of the present invention maycomprise optional components other than the aforementioned components.Examples of such optional components may include antifoaming agents,lubricants, ultraviolet absorbing agents, dyes, pigments, stabilizers,and surfactants. Moreover, as such optional components, organic ions mayalso be added to the cellulose fiber-containing composition.

Furthermore, resins other than the aforementioned thermoplastic resinmay also be added as such optional components. Examples of such resinsother than the thermoplastic resin include a thermosetting resin and aphotocurable resin. These resins may also be added in the form of anemulsion. Specific examples of such a thermosetting resin emulsion and aphotocurable resin emulsion include those described in JPA-2009-299043.

(Sheet)

The present invention also relates to a sheet formed from theaforementioned cellulose fiber-containing composition. The sheet of thepresent invention comprises cellulose fibers having a fiber width of1000 nm or less, and an antiseptic containing at least one type selectedfrom a nitrogen atom, a sulfur atom and a halogen atom. Herein, thecontent of the antiseptic is 10% by mass or less, with respect to thetotal mass of the sheet. The content of the antiseptic is preferably 5parts by mass or less, more preferably 3 parts by mass or less, evenmore preferably 1 part by mass or less, further preferably 0.5 parts bymass or less, particularly preferably 0.2 parts by mass or less, andmost preferably 0.1 part by mass or less, with respect to the total massof the sheet. On the other hand, the content of the antiseptic ispreferably 0.0001 part by mass or more, with respect to the total massof the sheet.

The tensile elastic modulus of the sheet of the present invention may be2.5 GPa or more, and it is preferably 3.0 GPa or more, and morepreferably 3.5 GPa or more. The upper limit value of the tensile elasticmodulus of the sheet is not particularly limited, and it can be set at,for example, 50 GPa or less. Even in a case where the sheet of thepresent invention is formed from a slurry for use in sheet formationthat has been left at rest at 50° C. for 192 hours, the tensile elasticmodulus of the sheet preferably satisfies the above-describedconditions.

Herein, the tensile elastic modulus of the sheet is a value measured inaccordance with JIS P 8113, using a tension testing machine “Tensilon”(manufactured by A & D Company, Limited). Upon the measurement of thetensile elastic modulus, a test piece to be measured was prepared byhumidity conditioning for 24 hours at 23° C. and a relative humidity of50%, and the measurement was then carried out under conditions of 23° C.and a relative humidity of 50%.

The haze of the sheet of the present invention is preferably 2% or less,more preferably 1.5% or less, and further preferably 1% or less.Moreover, the total light transmittance of the sheet of the presentinvention is preferably 85% or more, more preferably 90% or more, andfurther preferably 91% or more. Even in a case where the sheet of thepresent invention is formed from a slurry for use in sheet formationthat has been left at rest at 50° C. for 192 hours, the haze and thetotal light transmittance of the sheet preferably satisfy theabove-described conditions.

Herein, the total light transmittance and the haze of the sheet arevalues measured in accordance with JIS K 7136, using a hazemeter(manufactured by MURAKAMI COLOR RESEARCH LABORATORY Co., Ltd.; HM-150).

The thickness of the sheet of the present invention is not particularlylimited, and it is preferably 5 μm or more, more preferably 10 μm ormore, and further preferably 20 μm or more. The upper limit value of thethickness of the sheet is not particularly limited, and it can be setat, for example, 1000 μm or less. Besides, the thickness of the sheetmay be measured using a stylus thickness gauge (manufactured by Mahr;Millitron 1202 D).

The basis weight of the sheet of the present invention is preferably 10g/m² or more, more preferably 20 g/m² or more, and further preferably 30g/m² or more. On the other hand, the basis weight of the sheet ispreferably 100 g/m² or less, and more preferably 80 g/m² or less.Herein, the basis weight of the sheet may be calculated in accordancewith JIS P 8124.

(Method for Producing Sheet)

The step of producing a sheet comprises a step of obtaining a slurrycomprising ultrafine cellulose fibers having a fiber width of 1000 nm orless and an antiseptic containing at least one type selected from anitrogen atom, a sulfur atom and a halogen atom, and a step of applyingthis slurry onto a base material, or a step of papermaking from theslurry. Herein, the antiseptic is added in an amount of 10% by mass orless with respect to the total mass of the sheet.

In the step of obtaining a slurry, the antiseptic containing at leastone type selected from a nitrogen atom, a sulfur atom and a halogen atommay be added in an amount of 10 parts by mass or less, preferably 5parts by mass or less, more preferably 3 parts by mass or less, evenmore preferably 1 part by mass or less, further preferably 0.5 parts bymass or less, particularly preferably 0.2 parts by mass or less, andmost preferably 0.1 part by mass or less, with respect to 100 parts bymass of ultrafine cellulose fibers comprised in the slurry.

In the step of obtaining a slurry, the antiseptic containing at leastone type selected from a nitrogen atom, a sulfur atom and a halogen atommay be added in an amount of 10 parts by mass or less, preferably 5parts by mass or less, more preferably 3 parts by mass or less, evenmore preferably 1 part by mass or less, further preferably 0.5 parts bymass or less, particularly preferably 0.2 parts by mass or less, andmost preferably 0.1 part by mass or less, with respect to 100 parts bymass of ultrafine cellulose fibers comprised in the slurry.

In the step of obtaining a slurry, it is preferable to further add ahydrophilic polymer to the slurry. In addition, other than thehydrophilic polymer, polyamine polyamide epihalohydrin or athermoplastic resin may also be added. Thus, by adding a hydrophilicpolymer and the like to the slurry, a sheet having excellenttransparency and mechanical strength can be formed.

<Coating Step>

The coating step is a step of applying the slurry obtained in the stepof obtaining a slurry onto a base material, drying the slurry to form asheet, and then detaching the sheet from the base material to obtain asheet. Use of a coating apparatus and a long base material cancontinuously produce sheets.

The quality of the base material used in the coating step is notparticularly limited. Although a base material having higher wettabilityto the slurry is preferable because shrinkage of the sheet or the likeupon drying is suppressed, it is preferable to select one from which asheet formed after drying can be easily detached. Of these, a resinplate or a metal plate is preferable, without particular limitation.Examples of the base material that can be used herein include resinplates such as acrylic plates, polyethylene terephthalate plates, vinylchloride plates, polystyrene plates, and polyvinylidene chloride plates;metal plates such as aluminum plates, zinc plates, copper plates, andiron plates; plates obtained by the oxidation treatment of surfacethereof; and stainless plates and brass plates.

When the slurry has a low viscosity and spreads on the base material inthe coating step, a damming frame may be fixed and used on the basematerial in order to obtain a sheet having a predetermined thickness andbasis weight. The quality of the damming frame is not particularlylimited, but it is preferable to select ones from which edges of thesheet adhere after drying can be easily detached. Of these, framesformed from resin plates or metal plates are preferable, withoutparticular limitation. Example thereof that can be used herein includeframes formed from resin plates such as acrylic plates, polyethyleneterephthalate plates, vinyl chloride plates, polystyrene plates, andpolyvinylidene chloride plates; from metal plates such as aluminumplates, zinc plates, copper plates, and iron plates; from platesobtained by the oxidation treatment of surface thereof; and fromstainless plates and brass plates.

Examples of a coater for applying the slurry that can be used hereininclude roll coaters, gravure coaters, die coaters, curtain coaters, andair doctor coaters. Die coaters, curtain coaters, and spray coaters arepreferable because more even thickness can be provided.

The coating temperature is not particularly limited, but it ispreferably 20° C. or higher and 45° C. or lower, more preferably 25° C.or higher and 40° C. or lower, and further preferably 27° C. or higherand 35° C. or lower. When the coating temperature is equal to or higherthan the above-described lower limit value, it is possible to easilyapply the slurry. When the coating temperature is equal to or lower thanthe above-described upper limit value, it is possible to suppressvolatilization of the dispersion medium upon coating.

In the coating step, it is preferable to apply the slurry so as toachieve a finished basis weight of the sheet that is 10 g/m² or more and100 g/m² or less, and preferably, 20 g/m² or more and 60 g/m² or less.By applying the slurry so as to achieve a basis weight that is withinthe above-described range, a sheet having excellent strength can beobtained.

The coating step preferably includes a step of drying the slurry appliedonto the base material. The drying method is not particularly limited,but any of a contactless drying method and a method of drying the sheetwhile locking the sheet may be used, or these methods may also be usedin combination.

The contactless drying method is not particularly limited, but a methodfor drying by heating with hot air, infrared radiation, far-infraredradiation, or near-infrared radiation (a drying method by heating) or amethod for drying in vacuum (a vacuum drying method) can be utilized.Although the drying method by heating and the vacuum drying method maybe combined, the drying method by heating is usually utilized. Thedrying with infrared radiation, far-infrared radiation, or near-infraredradiation can be performed using an infrared apparatus, a far-infraredapparatus, or a near-infrared apparatus without particular limitations.The heating temperature for the drying method by heating is notparticularly limited, but it is preferably 20° C. or higher and 150° C.or lower, and more preferably 25° C. or higher and 105° C. or lower. Atthe heating temperature equal to or higher than the above-describedlower limit value, the dispersion medium can be rapidly volatilized. Atthe heating temperature equal to or lower than the above-described upperlimit value, cost required for the heating can be reduced, and thethermal discoloration of the ultrafine cellulose fibers can besuppressed.

<Papermaking Step>

The step of producing a sheet may include a step of papermaking from aslurry. Examples of a paper machine used in the papermaking step includecontinuous paper machines such as a Fourdrinier paper machine, acylinder paper machine, and an inclined paper machine, and a multilayercombination paper machine, which is a combination thereof. Knownpapermaking such as papermaking by hand may be carried out in thepapermaking step.

In the papermaking step, the slurry is wire-filtered and dehydrated toobtain a sheet that is in a wet state. The sheet is then pressed anddried to obtain a sheet. Upon filtration and dehydration of the slurry,a filter fabric for filtration is not particularly limited. It isimportant that ultrafine cellulose fibers or antiseptics do not passthrough the filter fabric and the filtration speed is not excessivelyslow. Such filter fabric is not particularly limited, and a sheetconsisting of an organic polymer, a woven fabric, or a porous membraneis preferable. Preferred examples of the organic polymer include, butare not particularly limited to, non-cellulose organic polymers such aspolyethylene terephthalate, polyethylene, polypropylene, andpolytetrafluoroethylene (PTFE). Specific examples thereof include, butare not particularly limited to, a polytetrafluoroethylene porousmembrane having a pore size of 0.1 μm or more and 20 μm or less, forexample, 1 μm, and woven fabric made of polyethylene terephthalate orpolyethylene having a pore size of 0.1 μm or more and 20 μm or less, forexample, 1 μm.

A method for producing a sheet from a slurry is not particularlylimited, but an example thereof is the method disclosed in WO2011/013567 comprising using a production apparatus. This productionapparatus comprises a dewatering section for ejecting an ultrafinecellulose fiber-containing slurry onto the upper surface of an endlessbelt and then dewatering a dispersion medium contained in the ejectedslurry to form a web, and a drying section for drying the web to producea fiber sheet. The endless belt is provided across from the dewateringsection to the drying section, and the web formed in the dewateringsection is transferred to the drying section while being placed on theendless belt.

The dehydration method that can be adopted in the present invention isnot particularly limited. An example of the method is a dehydrationmethod conventionally used for paper production. A preferred example isa method comprising performing dehydration using a Fourdrinier,cylinder, tilted wire, or the like and then performing dehydration usinga roll press. In addition, a drying method is not particularly limited,but an example thereof is a method used for paper production and forexample a method using a cylinder dryer, a yankee dryer, hot air drying,a near-infrared heater, or an infrared heater is preferable.

(Laminate)

A laminate may be formed by further laminating an additional layer onthe sheet obtained in the aforementioned step. Such an additional layermay be provided on both surfaces of the sheet, or may also be providedon one surface of the sheet. Examples of the additional layer that islaminated on at least one surface of the sheet may include a resin layerand an inorganic layer.

Specific examples of the laminate may include a laminate in which aresin layer is directly laminated on at least one surface of a sheet, alaminate in which an inorganic layer is directly laminated on at leastone surface of a sheet, a laminate in which a resin layer, a sheet andan inorganic layer are laminated in this order, a laminate in which asheet, a resin layer and an inorganic layer are laminated in this order,and a laminate in which a sheet, an inorganic layer and a resin layerare laminated in this order. The layer configuration of the laminate isnot limited to the above-described examples, and the laminate can havevarious aspects depending on intended use.

<Resin Layer>

The resin layer is a layer that has a natural resin or a synthetic resinas a main component. In this context, the main component refers to acomponent comprised in 50% by mass or more, based on the total mass ofthe resin layer. The content of the resin is preferably 60% by mass ormore, more preferably 70% by mass or more, further preferably 80% bymass or more, and particularly preferably 90% by mass or more, based onthe total mass of the resin layer. It is to be noted that the content ofthe resin may be set at 100% by mass, or may also be set at 95% by massor less.

Examples of natural resins may include rosin-based resins, such asrosin, rosin ester and hydrated rosin ester.

The synthetic resin is preferably at least one selected from, forexample, polycarbonate resins, polyethylene terephthalate resins,polyethylene naphthalate resins, polyethylene resins, polypropyleneresins, polyimide resins, polystyrene resins, polyurethane resins andacrylic resins. Among them, the synthetic resin is preferably at leastone selected from polycarbonate resins and acrylic resins, and morepreferably a polycarbonate resin. It is to be noted that the acrylicresin is preferably at least any one selected from polyacrylonitrile andpoly(meth)acrylate.

Examples of the polycarbonate resin, which constitutes the resin layer,include aromatic polycarbonate-based resins and aliphaticpolycarbonate-based resins. These specific polycarbonate-based resinsare known, and a polycarbonate-based resin described in JP-A-2010-023275is included, for example.

One resin that constitutes the resin layer may be used alone, or acopolymer obtained by copolymerization or graft polymerization of aplurality of resin components may be used. Alternatively, a plurality ofresin components may be mixed by a physical process and used as a blendmaterial.

An adhesive layer may be provided between the sheet and the resin layer,or the sheet and the resin layer may directly adhere to each otherwithout providing an adhesive layer. When an adhesive layer is providedbetween the sheet and the resin layer, examples of adhesives, whichconstitute the adhesive layer, may include acrylic resins. Examples ofadhesives other than acrylic resins include vinyl chloride resins,(meth)acrylic acid ester resins, styrene/acrylic acid ester copolymerresins, vinyl acetate resins, vinyl acetate/(meth)acrylic acid estercopolymer resins, urethane resins, silicone resins, epoxy resins,ethylene/vinyl acetate copolymer resins, polyester-based resins,polyvinyl alcohol resins, ethylene vinyl alcohol copolymer resins, andrubber-based emulsions such as SBR and NBR.

When no adhesive layer is provided between the sheet and the resinlayer, the resin layer may have an adhesion aid, or the surface of theresin layer may be surface-treated by a hydrophilization treatment orthe like.

Examples of the adhesion aid may include compounds containing at leastone selected from an isocyanate group, a carbodiimide group, an epoxygroup, an oxazoline group, an amino group and a silanol group, andorganic silicon compounds. Among them, the adhesion aid is preferably atleast one selected from a compound containing an isocyanate group(isocyanate compound) and an organic silicon compound. Examples of theorganic silicon compound may include silane coupling agent condensatesand silane coupling agents.

Examples of the surface treatment method other than the hydrophilictreatment may include a corona treatment, a plasma discharge treatment,a UV irradiation treatment, an electron beam irradiation treatment, anda flame treatment.

<Inorganic Layer>

Substances constituting the inorganic layer are not particularlylimited, but examples thereof include aluminum, silicon, magnesium,zinc, tin, nickel, and titanium; oxides, carbides, nitrides,oxycarbides, oxynitrides, and oxycarbonitrides thereof; and mixturesthereof. From the viewpoint that high moisture resistance can be stablymaintained, silicon oxide, silicon nitride, silicon oxycarbide, siliconoxynitride, silicon oxycarbonitride, aluminum oxide, aluminum nitride,aluminum oxycarbide, aluminum oxynitride, or mixtures thereof arepreferable.

A method for forming an inorganic layer is not particularly limited. Ingeneral, methods of forming a thin film are roughly classified intoChemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD),either of which may be employed. Specific examples of CVD methodsinclude plasma CVD, which utilizes plasma, and Catalyst Chemical VaporDeposition (Cat-CVD) including catalytically cracking material gas usinga heated catalyzer. Specific examples of PVD methods include vacuumdeposition, ion plating, and sputtering.

As a method for forming an inorganic layer, Atomic Layer Deposition(ALD) can also be employed. The ALD method is a method for forming athin film in an atomic layer unit by alternately supplying each ofsource gases of elements constituting the film to be formed to thesurface on which a layer is to be formed. This method, albeitdisadvantageous in a slow deposition rate, can more smoothly cover evena surface having a complicated shape than the plasma CVD method and hasthe advantage that a thin film having fewer defects can be formed. TheALD method also has the advantage that this method can control a filmthickness at a nano order and can relatively easily cover a widesurface, for example. The ALD method can be further expected to improvea reaction rate, to achieve a low-temperature process, and to decreaseunreacted gas, by using plasma.

(Intended Use)

The cellulose fiber-containing composition of the present invention canbe used, for example, as a thickener for various intended uses. Inaddition, the present cellulose fiber-containing composition is mixedwith a resin or an emulsion, so that it can also be used as areinforcing material. Otherwise, a slurry of the cellulosefiber-containing composition may be used to form a film, and thus,various types of sheets may be produced.

Such a sheet is suitable for intended uses, such as light transmissivesubstrates for various display devices, various solar cells, and thelike. In addition, the sheet of the present invention is also suitablefor intended uses, such as substrates of electronic devices, componentsof consumer electronics, window materials of various types of vehiclesor buildings, interior materials, exterior materials, and wrappingmaterials. Moreover, the sheet of the present invention is also suitablefor intended uses, such as threads, filters, woven fabrics, bufferingmaterials, sponges, and polishing materials, and also, other intendeduses, in which the sheet itself is used as a reinforcing material.

EXAMPLES

The characteristics of the present invention will be more specificallydescribed in the following examples and comparative examples. Thematerials, used amounts, ratios, treatment contents, treatmentprocedures, etc. can be appropriately modified, unless they are deviatedfrom the gist of the present invention. Accordingly, the scope of thepresent invention should not be restrictively interpreted by thefollowing specific examples.

Example 1 <Production of Phosphoric Acid Group-Introduced CelluloseFibers>

Pulp manufactured by Oji Paper Co., Ltd. (solid content: 93% by mass,basis weight: 208 g/m², sheet-shaped, Canadian Standard Freeness (CSF)measured according to JIS P 8121 after defibration: 700 ml) was used asneedle bleached kraft pulp. 100 Parts by mass (absolute dry mass) of theneedle bleached kraft pulp were impregnated with a mixed aqueoussolution of ammonium dihydrogen phosphate and urea, and were thencompressed to result in 49 parts by mass of the ammonium dihydrogenphosphate and 130 parts by mass of the urea, so as to obtainchemical-impregnated pulp. The obtained chemical-impregnated pulp wasdried in a dryer of 105° C. for moisture evaporation to pre-dry thechemical-impregnated pulp. Then, the chemical-impregnated pulp washeated in an air-blow dryer set at 140° C. for 10 minutes, so that aphosphoric acid group was introduced into cellulose in the pulp toobtain phosphorylated pulp. 10000 Parts by mass of ion exchange waterwere poured onto 100 parts by mass (absolute dry mass) of the obtainedphosphorylated pulp, which was then uniformly dispersed by stirring,followed by filtration and dehydration to obtain a dehydrated sheet.This step was repeated twice to obtain phosphoric acid modifiedcellulose fibers. Subsequently, 5000 ml of ion exchange water was addedto the cellulose into which the phosphoric acid group had beenintroduced, and the resultant mixture was stirred and washed, and thendehydration was carried out. The dehydrated pulp was diluted with 5000ml of ion exchange water, and a 1 N aqueous solution of sodium hydroxidewas gradually added, while stirring, until the pH became 12 or more and13 or less, so as to obtain a pulp dispersion. Then, this pulpdispersion was dehydrated and washed with 5000 ml of ion exchange water.This dehydration and washing was repeated one more time. The amount ofphosphoric acid groups introduced into the obtained phosphoric acidmodified cellulose fibers was 1.29 mmol/g. In addition, the obtainedphosphoric acid modified cellulose fibers had a fiber width ofapproximately 4 to 20 nm.

<Mechanical Treatment>

Ion exchange water was added to the pulp obtained after the washing anddehydration to produce a pulp suspension having a solid concentration of1.0% by mass. This pulp suspension was treated using a wet atomizationapparatus (manufactured by Sugino Machine Limited; Ultimizer,) at apressure of 245 MPa five times to obtain an ultrafine cellulose fibersuspension.

<Dissolving of Polyethylene Oxide>

Polyethylene oxide (manufactured by SUMITOMO SEIKA CHEMICALS CO., LTD.;PEO-18P) was added to ion exchange water to result in an amount of 2% bymass, and the mixture was then stirred, so that polyethylene oxide wasdissolved therein.

<Antiseptic>

As antiseptics, SLC series (SLC795 and SLC755) and MLN series (MLN352,MLN460 and MLN553P), which are manufactured by KATAYAMA NALCO INC., wereused. Hereafter, Antiseptic 1 indicates SLC795, Antiseptic 2 indicatesSLC755, Antiseptic 3 indicates MLN352, Antiseptic 4 indicates MLN460,and Antiseptic 5 indicates MLN553P.

TABLE 1 Product name Component Antiseptic 1 SLC7952,2-Dibromo-3-nitrilopropionamide Cyclic organic sulfur compound SolventAntiseptic 2 SLC755 2,2-Dibromo-3-nitrilopropionamide Organic nitrogensulfur compound Solvent Antiseptic 3 MLN3522,2-Dibromo-3-nitrilopropionamide Organic nitrogen sulfur compoundSolvent Antiseptic 4 MLN460 Solvent Organic nitrogen bromine compound —Antiseptic 5 MLN553P Ethylene glycol Cyclic organic nitrogen sulfurcompound —

Besides, 2,2-dibromo-3-nitrilopropionamide shown in Table 1 isclassified into the organic nitrogen halogen compound.

<Preparation of Slurry>

A polyethylene oxide (PEO) solution and Antiseptic 1 were added to anultrafine cellulose fiber suspension to prepare a slurry (ultrafinecellulose fiber-containing slurry), in which 20 parts by mass ofpolyethylene oxide and 0.0024 parts by mass of Antiseptic 1 could beused with respect to 100 parts by mass of ultrafine cellulose fibers.Besides, the concentration of the solid in the slurry was adjusted to be1.8% by mass.

<Sheet Formation>

The concentration of a solid in the slurry was adjusted to be 0.6% bymass. This slurry was divided into three vessels, and one of the threevessels was subjected to sheet formation without being left at rest at50° C. Two other vessels were left at rest at 50° C., and after 96 hoursor 192 hours later had passed, the vessels were subjected to theprocedures of sheet formation. In such sheet formation, each slurry wasweighed so that the finished basis weight of the sheet became 50 g/m²,was then developed onto a commercially available acrylic plate, and wasthen dried with a dryer at 70° C. for 24 hours. Here, a plate fordamming was arranged on the acrylic plate so as to have a predeterminedbasis weight. As a result of the above procedures, a sheet was obtained,and its thickness was 30 μm.

Example 2

A slurry was obtained in the same manner as that of Example 1, with theexception that Antiseptic 1 was changed to Antiseptic 2.

Example 3

A slurry was obtained in the same manner as that of Example 1, with theexception that Antiseptic 1 was changed to Antiseptic 3.

Example 4

A slurry was obtained in the same manner as that of Example 1, with theexceptions that Antiseptic 1 was changed to Antiseptic 4, and that theadditive amount of Antiseptic 4 was set at 0.012 parts by mass.

Example 5

A slurry was obtained in the same manner as that of Example 4, with theexception that the additive amount of Antiseptic 4 was set at 0.06 partsby mass.

Example 6

A slurry was obtained in the same manner as that of Example 4, with theexception that the additive amount of Antiseptic 4 was set at 0.18 partsby mass.

Example 7

A slurry was obtained in the same manner as that of Example 1, with theexceptions that Antiseptic 1 was changed to Antiseptic 5, and that theadditive amount of Antiseptic 5 was set at 0.024 parts by mass.

Example 8

A slurry was obtained in the same manner as that of Example 7, with theexception that the additive amount of Antiseptic 5 was set at 0.096parts by mass.

Example 9

A slurry was obtained in the same manner as that of Example 7, with theexception that the additive amount of Antiseptic 5 was set at 0.18 partsby mass.

Example 10 <Dissolving of Polyamine Polyamide Epichlorohydrin (PAE)>

Polyamine polyamide epichlorohydrin (manufactured by SEIKO PMCCORPORATION; Wet Strength Agent WS4030) was added to ion exchange waterto result in an amount of 1.8% by mass, and the mixture was thenstirred, so that polyamine polyamide epichlorohydrin was dissolvedtherein.

<Production of Polypropylene (PP) Emulsion>

Polypropylene (manufactured by TOHO Chemical Industry Co., Ltd.; HYTECP-5060P) was added to ion exchange water to result in an amount of 1.8%by mass, and the mixture was then stirred, so that polypropylene wasdissolved therein.

A polyethylene oxide (PEO) solution, a polyamine polyamideepichlorohydrin (PAE) solution, a polypropylene (PP) emulsion, andAntiseptic 1 were added to an ultrafine cellulose fiber suspension, soas to prepare a slurry, in which 20 parts by mass of polyethylene oxide,0.5 parts by mass of polyamine polyamide epichlorohydrin, 10 parts bymass of polypropylene, and 0.0024 parts by mass of Antiseptic 1 could beused with respect to 100 parts by mass of ultrafine cellulose fibers.Besides, the concentration of a solid in the slurry was adjusted to be1.8% by mass.

Upon sheet formation, the concentration was adjusted, so that the solidconcentration became 0.6% by mass. This slurry was divided into threevessels, and one of the three vessels was subjected to sheet formationwithout being left at rest at 50° C. Two other vessels were left at restat 50° C., and after 96 hours or 192 hours had passed, the vessels weresubjected to the procedures of sheet formation. In such sheet formation,each slurry was weighed so that the finished basis weight of the sheetbecame 50 g/m², was then developed onto a commercially available acrylicplate, and was then dried with a dryer at 70° C. for 24 hours. Here, aplate for damming was arranged on the acrylic plate so as to have apredetermined basis weight. As a result of the above procedures, a sheetwas obtained, and its thickness was 30 μm.

Example 11

A slurry and a sheet were obtained in the same manner as that of Example10, with the exception that Antiseptic 1 was changed to Antiseptic 2.

Example 12

A slurry was obtained in the same manner as that of Example 10, with theexception that Antiseptic 1 was changed to Antiseptic 3.

Example 13

A slurry was obtained in the same manner as that of Example 10, with theexceptions that Antiseptic 1 was changed to Antiseptic 4, and that theadditive amount of Antiseptic 4 was set at 0.012 parts by mass.

Example 14

A slurry was obtained in the same manner as that of Example 13, with theexception that the additive amount of Antiseptic 4 was set at 0.06 partsby mass.

Example 15

A slurry was obtained in the same manner as that of Example 13, with theexception that the additive amount of Antiseptic 4 was set at 0.18 partsby mass.

Example 16

A slurry was obtained in the same manner as that of Example 10, with theexceptions that Antiseptic 1 was changed to Antiseptic 5, and that theadditive amount of Antiseptic 5 was set at 0.024 parts by mass.

Example 17

A slurry was obtained in the same manner as that of Example 16, with theexception that the additive amount of Antiseptic 5 was set at 0.096parts by mass.

Example 18

A slurry was obtained in the same manner as that of Example 16, with theexception that the additive amount of Antiseptic 5 was set at 0.18 partsby mass.

Example 19

Undried needle bleached kraft pulp corresponding to a dry mass of 100parts by mass, 1.6 parts by mass of TEMPO, and 10 parts by mass ofsodium bromide were dispersed in 10000 parts by mass of water.Subsequently, an aqueous solution containing 13% by mass of sodiumhypochlorite was added thereto, such that the amount of sodiumhypochlorite became 3.5 mmol with respect to 1.0 g of the pulp, to startreaction. During the reaction, the pH was kept at pH 10 or more and pH11 or less by the dropwise addition of a 1.0 M sodium hydroxide aqueoussolution. The point in time when change in pH was no longer seen wasconsidered to be termination of the reaction, and carboxyl groups wereintroduced into the pulp. Thereafter, this pulp slurry was dehydrated toobtain a dehydrated sheet, and 5000 parts by mass of ion exchange waterwere poured onto the pulp, which was then uniformly dispersed bystirring, and then, filtration and dehydration were performed on theresultant to obtain a dehydrated sheet. This step was repeated twice, soas to obtain carboxyl group modified cellulose fibers. The amount ofcarboxyl groups introduced into the obtained carboxyl group modifiedcellulose fibers was 1.01 mmol/g. A slurry was obtained in the samemanner as that of Example 6, with the exception that the obtainedcarboxyl group modified cellulose fibers were used as ultrafinecellulose fibers.

Example 20

A slurry was obtained in the same manner as that of Example 1, with theexception that a polyethylene oxide (PEO) solution was not added in<Preparation of slurry> in Example 1.

Example 21

A slurry was obtained in the same manner as that of Example 20, with theexception that Antiseptic 1 was changed to Antiseptic 2.

Example 22

A slurry was obtained in the same manner as that of Example 20, with theexception that Antiseptic 1 was changed to Antiseptic 3.

Example 23

A slurry was obtained in the same manner as that of Example 20, with theexceptions that Antiseptic 1 was changed to Antiseptic 4, and that theadditive amount of Antiseptic 4 was set at 0.18 parts by mass.

Example 24

A slurry was obtained in the same manner as that of Example 20, with theexceptions that Antiseptic 1 was changed to Antiseptic 5, and that theadditive amount of Antiseptic 5 was set at 0.18 parts by mass.

Comparative Example 1

A slurry was obtained in the same manner as that of Example 1, with theexception that an antiseptic was not added.

Comparative Example 2

A slurry was obtained in the same manner as that of Example 10, with theexception that an antiseptic was not added.

Comparative Example 3

A slurry was obtained in the same manner as that of Example 4, with theexception that the additive amount of Antiseptic 4 was set at 50 partsby mass.

Comparative Example 4

A slurry was obtained in the same manner as that of Example 10, with theexception that the additive amount of Antiseptic 4 was set at 50 partsby mass.

Comparative Example 5

A slurry was obtained in the same manner as that of Example 19, with theexception that an antiseptic was not added.

[Evaluation]

The slurries and sheets produced in Examples and Comparative Exampleswere evaluated according to the following evaluation methods.

(1) Measurement of Amount of Substituent on Surface of Cellulose(Titration Method)

The amount of the phosphoric acid group introduced was measured bydiluting the cellulose with ion exchange water to a content of 0.2% bymass, then treating with an ion exchange resin, and titrating withalkali. In the treatment with the ion exchange resin, 1/10 by volume ofa strongly acidic ion exchange resin (manufactured by OrganoCorporation; Amberjet 1024; conditioned) was added to a slurrycontaining 0.2% by mass of the cellulose, and the resultant mixture wasshaken for 1 hour. Then, the mixture was poured onto a mesh having 90-μmapertures to separate the resin from the slurry. In the alkalititration, the change in the electric conductivity value indicated bythe slurry was measured while adding a 0.1 N aqueous solution of sodiumhydroxide to the slurry containing cellulose fibers after the ionexchange. Specifically, the alkali amount (mmol) required in the firstregion of the curve shown in FIG. 1 was divided by the solid content (g)in the slurry to be titrated, and the obtained value was taken as theamount (mmol/g) of the substituent introduced.

With regard to the amount of the carboxyl group introduced, the alkaliamount (mmol) required in the first region of the curve shown in FIG. 2(carboxyl group) was divided by the solid content (g) in the slurry tobe titrated, and the obtained value was taken as the amount (mmol/g) ofthe substituent introduced.

(2) Measurement of Total Light Transmittance

The obtained slurry was left at rest at 50° C., and then, 1 day, 2 days,4 days, 8 days, 16 days, or 20 days later, the total light transmittancethereof was measured. On the other hand, the total light transmittanceof a slurry that had not been left at rest at 50° C. (50° C., 0 day) wasalso measured. In addition, the total light transmittance of each of thethree types of sheets (time for leaving at rest at 50° C.: 0 hour, 96hours, or 192 hours) obtained in Examples and Comparative Examples wasalso measured.

The total light transmittance of a sheet was measured in accordance withJIS K 7361, using a hazemeter (manufactured by MURAKAMI COLOR RESEARCHLABORATORY Co., Ltd.; HM-150). The total light transmittance of a slurrywas measured by leaving the slurry at 50° C. for the aforementionedperiod of time and then by setting the solid concentration of the slurryat 0.2% by mass, according to JIS K 7361. Upon the measurement, theslurry was placed in a glass cell for liquid having an optical pathlength of 1 cm (manufactured by Fujiwara Scientific Co., Ltd.; MG-40;inverse optical path) and was then measured. The zero point was measuredwith ion exchange water which was placed in the glass cell.

(3) Measurement of Haze

The obtained slurry was left at rest at 50° C., and then, 1 day, 2 days,4 days, 8 days, 16 days, or 20 days later, the haze thereof wasmeasured. On the other hand, the haze of a slurry that had not been leftat rest at 50° C. (50° C., 0 day) was also measured. In addition, thehaze of each of the three types of sheets (time for leaving at rest at50° C.: 0 hour, 96 hours, or 192 hours) obtained in Examples andComparative Examples was also measured.

The haze of a sheet was measured in accordance with JIS K 7136, using ahazemeter (manufactured by MURAKAMI COLOR RESEARCH LABORATORY Co., Ltd.;HM-150). The haze of a slurry was measured by leaving the slurry at 50°C. for the aforementioned period of time and then by setting the solidconcentration of the slurry at 0.2% by mass, according to JIS K 7361.Upon the measurement, the slurry was placed in a glass cell for liquidhaving an optical path length of 1 cm (manufactured by FujiwaraScientific Co., Ltd.; MG-40; inverse optical path) and was thenmeasured. The zero point was measured with ion exchange water which wasplaced in the glass cell.

(4) Measurement of Viscosity

The obtained slurry was left at rest at 50° C., and then, 1 day, 2 days,4 days, 8 days, 16 days, or 20 days later, the viscosity thereof wasmeasured. On the other hand, the viscosity of a slurry that had not beenleft at rest at 50° C. (50° C., 0 day) was also measured. Themeasurement of viscosity was carried out using a type B viscometer (No.4 roter) (manufactured by BROOKFIELD; analog viscometer T-LVT) underconditions of 25° C., a rotation number of 3 rpm, and a measurement timeof 3 minutes. Besides, the solid concentration of the slurry, which wasused in the measurement of viscosity, was 1.8% by mass.

(5) Measurement of Tensile Properties

The tensile elastic modulus of each of the three types of sheets (timefor leaving at rest at 50° C.: 0 hour, 96 hours, or 192 hours) obtainedin Examples and Comparative Examples was measured.

The tensile elastic modulus was measured in accordance with JIS P 8113,using a tension testing machine “Tensilon” (manufactured by A & DCompany, Limited). Upon the measurement of the tensile elastic modulus,a test piece prepared by humidity conditioning for 24 hours at 23° C.and a relative humidity of 50% was used.

TABLE 2 Amount of Fiber functional group diameter Mixing ratio (part bymass) Content [mass % to sheet] Functional group introduced [mmol/g][nm] Antiseptic CNF PEO PAE PP Antiseptic CNF PEO Antiseptic Ex. 1Phosphoric acid group 1.29 4-20 Antiseptic 1 100 20 0 0 0.0024 83.3 16.70.0020 Ex. 2 Phosphoric acid group 1.29 4-20 Antiseptic 2 100 20 0 00.0024 83.3 16.7 0.0020 Ex. 3 Phosphoric acid group 1.29 4-20 Antiseptic3 100 20 0 0 0.0024 83.3 16.7 0.0020 Ex. 4 Phosphoric acid group 1.294-20 Antiseptic 4 100 20 0 0 0.012 83.3 16.7 0.0100 Ex. 5 Phosphoricacid group 1.29 4-20 Antiseptic 4 100 20 0 0 0.06 83.3 16.7 0.0500 Ex. 6Phosphoric acid group 1.29 4-20 Antiseptic 4 100 20 0 0 0.18 83.2 16.60.1498 Ex. 7 Phosphoric acid group 1.29 4-20 Antiseptic 5 100 20 0 00.024 83.3 16.7 0.0200 Ex. 8 Phosphoric acid group 1.29 4-20 Antiseptic5 100 20 0 0 0.096 83.3 16.7 0.0799 Ex. 9 Phosphoric acid group 1.294-20 Antiseptic 5 100 20 0 0 0.18 83.2 16.6 0.1498 Ex. 10 Phosphoricacid group 1.29 4-20 Antiseptic 1 100 20 0.5 10 0.0024 76.6 15.3 0.0018Ex. 11 Phosphoric acid group 1.29 4-20 Antiseptic 2 100 20 0.5 10 0.002476.6 15.3 0.0018 Ex. 12 Phosphoric acid group 1.29 4-20 Antiseptic 3 10020 0.5 10 0.0024 76.6 15.3 0.0018 Ex. 13 Phosphoric acid group 1.29 4-20Antiseptic 4 100 20 0.5 10 0.012 76.6 15.3 0.0092 Ex. 14 Phosphoric acidgroup 1.29 4-20 Antiseptic 4 100 20 0.5 10 0.06 76.6 15.3 0.0460 Ex. 15Phosphoric acid group 1.29 4-20 Antiseptic 4 100 20 0.5 10 0.18 76.515.3 0.1377 Ex. 16 Phosphoric acid group 1.29 4-20 Antiseptic 5 100 200.5 10 0.024 76.6 15.3 0.0184 Ex. 17 Phosphoric acid group 1.29 4-20Antiseptic 5 100 20 0.5 10 0.096 76.6 15.3 0.0735 Ex. 18 Phosphoric acidgroup 1.29 4-20 Antiseptic 5 100 20 0.5 10 0.18 76.5 15.3 0.1377 Ex. 19Carboxyl group 1.01 4-20 Antiseptic 4 100 20 0 0 0.18 83.2 16.6 0.1498Ex. 20 Phosphoric acid group 1.29 4-20 Antiseptic 1 100 0 0 0 0.0024 1000 0.0024 Ex. 21 Phosphoric acid group 1.29 4-20 Antiseptic 2 100 0 0 00.0024 100 0 0.0024 Ex. 22 Phosphoric acid group 1.29 4-20 Antiseptic 3100 0 0 0 0.0024 100 0 0.0024 Ex. 23 Phosphoric acid group 1.29 4-20Antiseptic 4 100 0 0 0 0.18 100 0 0.18 Ex. 24 Phosphoric acid group 1.294-20 Antiseptic 5 100 0 0 0 0.18 100 0 0.18 Comp. Phosphoric acid group1.29 4-20 None 100 20 0 0 0 83.3 16.7 0 Ex. 1 Comp. Phosphoric acidgroup 1.29 4-20 None 100 20 0.5 10 0 76.6 15.3 0 Ex. 2 Comp. Phosphoricacid group 1.29 4-20 Antiseptic 4 100 20 0 0 50 58.8 11.8 29.4 Ex. 3Comp. Phosphoric acid group 1.29 4-20 Antiseptic 4 100 20 0.5 10 50 55.411.1 27.7 Ex. 4 Comp. Carboxyl group 1.01 4-20 None 100 20 0 0 0 83.316.7 0 Ex. 5

TABLE 3 Slurry Total light transmittance retention Total lighttransmittance [%] percentage (%) Haze [%] Initiation of 1 2 4 8 16 20After 480 Initiation of 1 2 4 8 measurement day days days days days dayshours measurement day days days days Ex. 1 99.8 99.8 99.7 99.7 99.7 99.699.7 99.9 0.8 0.4 0.2 0.2 0.3 Ex. 2 99.8 99.8 99.8 99.8 99.7 99.6 99.699.8 0.8 0.4 0.2 0.2 0.3 Ex. 3 99.7 99.8 99.8 99.8 99.7 99.5 99.5 99.80.8 0.4 0.2 0.2 0.5 Ex. 4 99.7 99.6 99.7 99.7 99.5 99.6 99.6 99.9 0.80.5 0.7 0.7 1   Ex. 5 99.8 99.8 99.8 99.8 99.7 99.6 99.7 99.9 0.8 0.40.2 0.2 0.4 Ex. 6 99.9 99.8 99.8 99.7 99.6 99.5 99.5 99.6 0.8 0.4 0.30.3 0.4 Ex. 7 99.8 99.8 99.8 99.8 99.7 99.6 99.7 99.9 0.9 0.5 0.6 0.60.9 Ex. 8 99.8 99.9 99.9 99.8 99.9 99.7 99.6 99.8 0.6 0.3 0.2 0.2 0.2Ex. 9 99.8 99.8 99.9 99.7 99.8 99.6 99.6 99.8 0.6 0.2 0.2 0.2 0.2 Ex. 1099.7 99.7 99.8 99.8 99.6 99.6 99.6 99.9 0.5 0.5 0.4 0.4 0.3 Ex. 11 99.799.7 99.7 99.7 99.6 99.5 99.5 99.8 0.5 0.3 0.4 0.5 0.3 Ex. 12 99.6 99.799.8 99.7 99.6 99.5 99.5 99.9 0.6 0.5 0.5 0.6 0.5 Ex. 13 99.6 99.2 99.399.2 99.1 99.1 99.2 99.6 0.8 0.5 0.6 0.7 0.7 Ex. 14 99.6 93.6 99.7 99.699.6 99.5 99.6 100.0 0.7 0.5 0.5 0.5 0.4 Ex. 15 99.7 99.7 99.8 99.7 99.699.5 99.6 99.9 0.6 0.3 0.4 0.5 0.4 Ex. 16 99.7 99.7 99.7 99.7 99.6 99.499.5 99.8 0.8 0.5 0.5 0.7 0.8 Ex. 17 99.8 99.8 99.8 99.8 99.8 99.6 99.799.9 0.5 0.2 0.3 0.5 0.3 Ex. 18 99.7 99.7 99.7 99.7 99.7 99.7 99.8 100.10.6 0.4 0.8 0.6 0.8 Ex. 19 — — — — — — — — — — — — Ex. 20 99.8 99.8 99.899.7 99.7 99.8 99.8 100.0 0.6 0.4 0.5 0.5 0.4 Ex. 21 99.8 99.7 99.7 99.899.7 99.8 99.7 99.9 0.5 0.5 0.5 0.4 0.6 Ex. 22 99.7 99.6 99.7 99.7 99.899.7 99.6 99.9 0.5 0.5 0.4 0.5 0.5 Ex. 23 99.7 99.7 99.6 99.7 99.8 99.899.8 100.1 0.5 0.5 0.6 0.4 0.5 Ex. 24 99.8 99.8 99.8 99.7 99.8 99.7 99.799.9 0.5 0.5 0.5 0.6 0.5 Comp. Ex. 1 99.7 99.8 99.9 99.8 99.7 99.7 99.699.9 0.9 1.6 1.2 1.3 1.2 Comp. Ex. 2 99.7 99.7 99.8 99.8 99.7 99.7 99.7100.0 0.7 1.4 1.4 1.4 1.6 Comp. Ex. 3 — — — — — — — — — — — — — Comp.Ex. 4 — — — — — — — — — — — — — Comp. Ex. 5 — — — — — — — — — — — — —Slurry Haze Viscosity increasing retention percentage percentage Haze[%] (%) Viscosity [mPa · s] (%) 16 20 After 480 Initiation of 1 2 4 8 1620 After 480 days days hours measurement day days days days days dayshours Ex. 1 0.4 0.4 −50.0 82000 84000 88000 86000 88000 83000 82000100.0 Ex. 2 0.5 0.5 −37.5 82000 86000 85000 86000 84000 82000 82000100.0 Ex. 3 0.5 0.5 −37.5 82000 86000 85000 85000 86000 86000 86000104.9 Ex. 4 1 1   25.0 84000 87000 85000 86000 88000 84000 83000 98.8Ex. 5 0.4 0.4 −50.0 83000 84000 88000 85000 86000 85000 84000 101.2 Ex.6 0.4 0.4 −50.0 84000 88000 86000 86000 86000 87000 86000 102.4 Ex. 7 10.9 0.0 85000 85000 79000 84000 82000 80000 80000 94.1 Ex. 8 0.3 0.4−33.3 82000 86000 86000 85000 86000 82000 80000 97.6 Ex. 9 0.4 0.5 −16.783000 86000 80000 80000 82000 82000 82000 98.8 Ex. 10 0.5 0.5 0.0 7600068000 69000 68000 72000 72000 70000 92.1 Ex. 11 0.4 0.5 0.0 75000 6900067000 70000 72000 73000 72000 96.0 Ex. 12 0.6 0.6 0.0 76000 71000 7400072000 74000 73000 73000 96.1 Ex. 13 0.7 0.7 −12.5 76000 74000 7400080000 77000 76000 75000 98.7 Ex. 14 0.6 0.6 −14.3 74000 68000 7500072000 74000 74000 74000 100.0 Ex. 15 0.5 0.5 −16.7 74000 67000 7300074000 74000 72000 72000 97.3 Ex. 16 0.9 0.8 0.0 74000 68000 70000 7100079000 76000 74000 100.0 Ex. 17 0.5 0.5 0.0 76000 70000 76000 70000 7200072000 72000 94.7 Ex. 18 0.9 0.9 50.0 77000 67000 70000 70000 70000 7000069000 89.6 Ex. 19 — — 82000 83000 82000 83000 82000 83000 83000 101.2Ex. 20 0.5 0.5 −17 97000 96000 98000 97000 96000 96000 96000 99.0 Ex. 210.5 0.5 0 97000 98000 97000 98000 98000 96000 97000 100.0 Ex. 22 0.6 0.50 96000 96000 95000 96000 97000 97000 96000 100.0 Ex. 23 0.5 0.4 −2098000 98000 97000 97000 98000 97000 97000 99.0 Ex. 24 0.6 0.5 0 9700096000 97000 96000 96000 97000 97000 100.0 Comp. Ex. 1 1.3 1.2 33.3 8500058000 60000 64000 58000 58000 58000 68.2 Comp. Ex. 2 1.5 1.6 128.6 7600064000 67000 65000 67000 52000 53000 69.7 Comp. Ex. 3 — — — 63000 — — — —— — — Comp. Ex. 4 — — — 53000 — — — — — — — Comp. Ex. 5 — — — 8100062000 61000 63000 62000 62000 63000 77.8

TABLE 4 Sheet Tensile elastic modulus [GPa] Total light transmittance[%] Haze [%] Initiation of Initiation of Initiation of measurement 4days 8 days measurement 4 days 8 days measurement 4 days 8 days Example1 4.69 4.17 3.99 91.8 91.6 91.7 0.4 0.3 0.3 Example 6 4.17 4.12 4.5 91.191.7 91.7 0.5 0.2 0.2 Example 10 4.36 3.91 4.32 91.8 91.6 91.6 0.6 0.20.2 Example 11 3.99 4.57 3.86 91.7 91.7 91.7 0.4 0.3 0.3 Example 15 3.953.93 3.71 91.8 91.6 91.7 0.5 0.3 0.4 Example 18 3.74 4.02 3.57 91.3 91.791.8 0.5 0.3 0.3

Regarding the slurries obtained in Examples, the viscosity of the slurrywas maintained even after long-term storage. In addition, the slurriesobtained in Examples tended to maintain high total light transmittanceand low haze.

On the other hand, regarding the slurries obtained in ComparativeExamples 1 and 2, the viscosity was decreased after long-term storage,and further, an increase in the haze was observed. Regarding theslurries obtained in Comparative Examples 3 and 4, the initial viscositybecame low, and thickening properties were not sufficiently exhibited.

Moreover, the sheets obtained in Examples had high tensile elasticmodulus, high total light transmittance, and low haze, regardless of thetime of storing the slurries. On the other hand, the sheets obtained inComparative Examples tended to deteriorate in terms of transparency, asthe time of storing the slurry was prolonged.

1-12. (canceled)
 13. A cellulose fiber-containing composition comprisingcellulose fibers having a fiber width of 1000 nm or less, and anantiseptic containing at least one type selected from a nitrogen atom, asulfur atom and a halogen atom, wherein the content of the antiseptic is10 parts by mass or less with respect to 100 parts by mass of thecellulose fibers.
 14. The cellulose fiber-containing compositionaccording to claim 13, wherein the cellulose fibers have a phosphoricacid group or a phosphoric acid group-derived substituent.
 15. Thecellulose fiber-containing composition according to claim 13, whereinthe antiseptic is at least one type selected from an organic nitrogensulfur compound, an organic nitrogen halogen compound, an organicnitrogen compound, and an organic sulfur compound.
 16. The cellulosefiber-containing composition according to claim 13, which furthercomprises a hydrophilic polymer, wherein the content of the hydrophilicpolymer is 0.5 parts by mass or more and 50 parts by mass or less, withrespect to 100 parts by mass of the cellulose fibers.
 17. The cellulosefiber-containing composition according to claim 13, wherein thecellulose fiber-containing composition is a slurry, and when the solidconcentration of the slurry is set at 1.8% by mass, the viscositymeasured under conditions of 25° C. and a rotation number of 3 rpm is70000 mPa·s or more.
 18. The cellulose fiber-containing compositionaccording to claim 13, wherein the cellulose fiber-containingcomposition is a slurry having a viscosity retention percentage of 80%or more, wherein the viscosity retention percentage is defined accordingto the following equation:Viscosity retention percentage (%)=slurry viscosity left at rest at 50°C. for 480 hours/initial slurry viscosity×100, provided that the slurryviscosity after being left at rest at 50° C. for 480 hours is aviscosity obtained by setting the solid concentration of the slurry at1.8% by mass and measuring under conditions of 25° C. and a rotationnumber of 3 rpm, after the slurry has been left at rest at 50° C. for480 hours, whereas the initial slurry viscosity is a viscosity obtainedby setting the solid concentration of the slurry at 1.8% by mass andmeasuring under conditions of 25° C. and a rotation number of 3 rpm. 19.The cellulose fiber-containing composition according to claim 13,wherein the cellulose fiber-containing composition is a slurry, and thehaze measured by setting the solid concentration of the slurry at 0.2%by mass and using a glass cell having an optical path length of 1 cm inaccordance with JIS K 7136 is 2.0% or less.
 20. The cellulosefiber-containing composition according to claim 13, wherein thecellulose fiber-containing composition is a slurry having a hazeincreasing percentage of 50% or less, wherein the haze increasingpercentage is defined according to the following equation:Haze increasing percentage (%)=(haze of slurry after being left at restat 50° C. for 480 hours−initial haze of slurry)/initial haze ofslurry×100, provided that the haze of a slurry after being left at restat 50° C. for 480 hours is a haze measured by setting the solidconcentration of the slurry at 0.2% by mass and using a glass cellhaving an optical path length of 1 cm in accordance with JIS K 7136,after the slurry has been left at rest at 50° C. for 480 hours, whereasthe initial haze of a slurry is a haze measured by setting the solidconcentration of the slurry at 0.2% by mass and using a glass cellhaving an optical path length of 1 cm in accordance with JIS K
 7136. 21.The cellulose fiber-containing composition according to claim 13,wherein the cellulose fiber-containing composition is a slurry for usein the formation of a sheet.
 22. The cellulose fiber-containingcomposition according to claim 13, wherein the cellulosefiber-containing composition is a slurry for use in the formation of asheet, wherein the sheet, that formed after the slurry has been left atrest at 50° C. for 192 hours, has a haze measured in accordance with JISK 7136 of 1% or less.
 23. A sheet comprising cellulose fibers having afiber width of 1000 nm or less and an antiseptic containing at least onetype selected from a nitrogen atom, a sulfur atom and a halogen atom,wherein the content of the antiseptic is 10% by mass or less withrespect to the total mass of the sheet.
 24. The sheet according to claim23, wherein the tensile elastic modulus measured under conditions of 23°C. and a relative humidity of 50% in accordance with JIS P 8113 is 3 GPaor more.